(The long answer)
This is not a simple cut and dry question but as you look at used boats they are likely to have been influenced by one racing rule or another. There are several types of rating rules and it is important to understand how each of these types of racing rules impacts the design of a boat. In the besty rules teh boast are designed beat the rules of the sea and to be good boats, and not to beat some man made rule. In a broad sense the most common types of were Measurement Rules, Developmental Rules, Performance handicapping, and Performance Prediction.
For most of the time that there has been racing there has been ''Measurement Rules''. These try to predict the performance of a boat based on a minimal number of measurements. If you only measure very specific items, or you over or under handicap a particular item, designers can exploit these loop holes to gain un-rated speed. Often this can have undesirable results. Take for example the first measurement rule which simply measured length. Designers quickly produced lighter weight boats with huge stability and sail plans.
When a rule has loopholes that shape the way that a boat is designed, this is called ?type forming?. Type forming is generally seen as a bad thing since it creates boats that are shaped to beat a rule rather than sail well, or fast, or comfortably, or be seaworthy. Measurement rules tend to promote extreme cases of type forming.
In the case of developmental rules, huge loopholes are left in the rule to allow the development of new technologies. A classic case of this would be the International Rule (not to be mistaken for the IOR- International Offshore Rule) which produced the former America''s Cup 12 meters. Another example of a development rule, is the Open Class rules (used by the single-handed around the distance racers) which allow almost anything as long as the boat is certain maximum length and has adequate stability. These boats are simply maximized for speed on the courses that they are expected to race.
Performance Handicapping looks at the actual performance of the boat in questions and assigns a rating to that boat based on how fast it sails. The shortcoming of this is that all boats do better or worse in specific types of conditions and the process is also subject to a lot of subjective interpretation. For example, suppose someone buys a relatively rare boat and starts winning a lot of races. The temptation is to alter the rating so that the boat wins less, but it may be that the boat is simply doing well because it is well sailed.
Lastly there is Performance Prediction based rating rules. Performance Prediction based rating rules look at a huge number of measurements and then attempt to predict how a boat will perform in a wide range of conditions. It will often include a component that adjusts for wind and sea state so that a boat with light air performance won?t always win in light air. In principle this should promote fair rating between boats of all different types and should not promote a boat type formed by the rule. The problem with Performance Prediction rating methods is that it requires a lot of data and sophisticated programs to accurately rate a boat. This is expensive. Also there is a certain amount of subjectivity in analyzing the wind and sea state conditions.
In terms specific rules, MORC, CCA, RORC and IOR are all mid to late 20th century measurement rules. Each distorted boat design tremendously and adversely. In the case of the CCA rule and RORC rule which were very similar, waterline length was over penalized producing boats with very short waterlines. This really hurt speed, seaworthiness and motion comfort. Jibs and mizzens were not taxed at all (or were hardly taxed near the end of the rule) forcing mainsails to get smaller an genoas to get much larger. Larger genoas were harder to handle and are comparatively inefficient. Masthead rigs came into being since they permitted huge amounts of untaxed sail areas, especially downwind. Yawls and ketches came into popular use as a way to beat the rule. The proportions of these rigs were not based on the traditional reason that these rigs had existed. Draft was over taxed so shoal draft keels and Centerboards had an untaxed advantage. Both hurt performance but not as much as the rig thought that they did.
While beautiful to look at CCA boats are miserable to sail. They tend to pitch and roll through wide angles. They have comparatively blunt bows that makes every collision with a wave?well?.a collision. They tend to be tender preferring to be sailed at large heel angles. They tend to be comparatively wet to sail. They are slow and tend to do poorly in lighter conditions. They really do not do well in a chop. They tend to have larger line loads and poorer hardware to handle it. They often had fin keel with attached rudders (which despite advertisements calling these full keels) offer none of the advantages of a full keel or a fin keel with most of the disadvantages of both. They often carry major weather helm and require more frequent sail changes which frankly can grind down a crew pretty quickly in changeable conditions.
The IOR was intended to correct the abuses of the CCA/RORC rules. The number of measurement points were increased. Stability was taxed for the first time. Because of the specificity of measurement points the IOR created distortions that in the produced boats that proved to be fatal to sail in heavy conditions. TO being with, the CCA rule produced boats that had long ends in order to produce more sailing length when heeled. To counteract that unhealthy trend, the IOR took a series of measurements around the bow and stern, these produced boats with very pinched ends. Beam was seen a slowing factor so the IOR type forms tended to be quite beamy. The IOR penalized stability so the IOR produced boats with high vertical center of gravities, narrow waterline beams, and eventually a lot of flare so that crew weight could be loaded on the rail. This combination of wide beam, excessive flare, pinched ends, and low stability produced boats that wanted to heel a lot but would jack their rudders out of the water when they heeled and would produce an asymmetric heeled waterplane so that they could and would quickly lose control when heeled. They also had a tendency to roll steer, meaning that they would change course pretty dramatically as they rolled from side to side unless a quick steering response was added but that steering response tended to increase the amount of rolling.
The hull form of an IOR typeform (especially later period boats) usually included comparatively flat sections, with a deep canoe body, pinched ends, a bustle near the rudder and comparatively short keel foils.
Like the previous rules, the IOR over penalized mainsails and under penalized jibs and spinnakers by a long shot. IOR boats pushed this to the absolute limit with excruciatingly small mainsails and enormous genoas and spinnakers. This is the opposite proportion that is idea for a cruising boat. Large foresails are much harder to handle and there was absolutely no aerodynamic reason for this rig proportion. In fact by the 1920?s, it was known that fractional rigs with minimally overlapping jibs offered the most drive per square foot of sail area and we now know that they offer the most flexibility in dealing with changing windspeeds. But the IOR rule had pushed the type form rig into a proportion that was harder to handle, required a larger sail inventory, placed greater stresses on the hull and was less seaworthy for a variety of reasons.
In the wake of the 1979 Fastnet Disaster designers and scientists began looking in earnest at the seaworthiness and seakindliness of race boats. In the ensuing research the IOR type form came under justified heavy criticism. Much of the impact of the research filtered into changes in the IOR rule and into thinking about what a race boat should be. This resulted in a rapid succession of IOR rule changes and a general disgust with the IOR rule and the boats they produced. There was also a realization that measurement rules were not a very good way to achieve good boats.
Another side issue of boats modeled after the IOR rule type form is that boat building technology allowed boats to get much lighter during the period in which the IOR rule was popular. In general it is displacement not length that dictates maintenance costs, ease of handling and the factors that we normally think of as describing the size of a vessel. There are a lot of good reasons why a longer boat for its weight is a good thing. BUT unfortunately many of these earlier light weight boats were built to the IOR model creating an erroneous sense that lighter boats are not as seaworthy or comfortable at sea.
With the advent of faster computers, Performance Prediction (also called VPP for velocity prediction programs) based rating systems became possible. The first was the MHS which later evolved into the IMS. In theory IMS should permit boats of different types to fairly against each other and should not produce a type form. In reality, if the speed advantages of all boats are fairly assessed and adjusted, then the faster boat will generally have a real advantage on the race course. This occurs for a lot of reasons. First of all the greater speed of the faster boat allows that boat to make tactic decisions about where it wants to be on the race course to experience the most optimal conditions and to sail a little further distance in order to sail a faster overall time. Also in equal winds the faster boat will experience greater apparent wind which allows it to leverage its speed advantage. Because of that, IMS has produced boats that are type formed for maximum speed. It has also produced boats that are easier to handle and can be sailed with smaller crews and boats that are well rounded in terms of performance on all points of sail and in a wide range of wind and sea states.
As an odd and very beneficial outcome of the IMS, there has been a real focus on motion and stability. Because quick abrupt motion and large roll and pitch angles, disrupt the flow over the sails, keel and rudder, there has been a real effort to improve the motion of yachts. This has resulted in race boats that often have better motion comfort than substantially heavier cruising types.
As a type form, IMS boats have close to a plumb stem, a very fine entry and fractional rigs often with non-or minimally overlapping jibs. They tend to have very low vertical center of gravities and moderate to minimal form stability. Their center of buoyancy occurs fairly far aft in the boat compared to mid 20th century designs. In a general sense the engineering is much better on these boats and although quite light they tend to be quite robust.
Now to the relevancy of all of this. To a great extent, builders of coastal cruisers who have tried to produce dual purpose boats (boats that could be raced or cruised) or have tried to produce boats that were styled to look fast, have modeled their boats after the race boats of the day. In as much as rules like the CCA or IOR produced pretty unwholesome boats, this is not very good news. In recent years, many boat builders have begun to learn from the technology trickling down from the IMS type forms and as such are producing increasing better boats. One disturbing trend is that some models are drawing off of the Open Class type forms. Open class boats tend to optimized for very high reaching speeds and for tremendous form and moveable ballast stability. To me this is a step backward in terms of motion comfort, ultimate stability, and ease of handling.