Bluewater passages aren't necessarily any more difficult than coastal ones, but they do require a different mindset and skill set. Also, open ocean passages are very different from coastal cruising, and each has different problems.
On a bluewater passage, you have a bit less to worry about in terms of collision risks, as there is generally far less traffic on the open ocean. You also don't have to worry about groundings, sandbars, rocks, lee shores, or other land-based issues as much.
Weather becomes much more of an issue, as you don't have any where to hide. In some ways, unless you're caught by a really fierce storm, being on the open ocean is far safer than getting caught in the same storm along the coast. The really strong storms, like hurricanes, North Atlantic gales, and such are truly a test for the bluewater sailor though. Even getting caught on the fringes of one of these can test your skills and your boat.
Navigation becomes more of an issue, especially if you're using GPS for it, as you really need to have and use manual backups, in case of failure, as there is no real way to determine your position on the open ocean, other than with a sextant, if your electronics fail.
Dead reckoning and a good understanding of the currents, leeway and such all becomes much more important. If you're 3000 miles from an island, like Hawaii, being a little off on the navigation can mean you'll miss it completely—currents, leeway, compass errors, etc, can all make it all to easy to miss an 100 mile-wide target at the end of 3000 miles.
Granted, if your GPS doesn't fail, and you don't have electrical problems, then this is not much of an issue, but going bluewater, you need to be ready for those types of failures and problems.
Floating hazards are a bit more of an issue, as floating hazards near populated areas are usually quickly dealt with—on the open ocean, there is no one to deal with them. Also, there are fewer people to spot them or warn you about them. In coastal waters, before a floating hazard is removed, you'll usually hear a "Securité" call about it, warning marine traffic to its prescence.
Understanding ocean swells and wind fetch generally becomes much more important, as the distances the waves build over is far greater—you can be seriously affected by a storm that is hundreds of miles away.
Some people don't deal well with being out of sight of land. The motion of the boat is a bit different than it is near the coast. You have to be more self-reliant as help is not a short distance away, but possible hours or days away. If you don't have an satellite phone, SSB-radio or an EPIRB, often you are going to have to be very self-reliant, as you won't even have any way of calling for help.
That said, the winds on the open ocean are generally far more constant than they are near the coast, where the land and buildings can affect the direction of the wind. A good solid self-steering system is often considered a necessity for long off-shore passages generally.
Wind vanes are nice because their ability to steer in bad conditions is generally far better than autopilots—as the wind strength increases, so does the power of a wind vane steering. They also don't use any electricity. That said, the wind can shift and if your not paying attention, the wind vane can carry you a long distance in the wrong direction.
Autopilots are a good complement to windvanes. They work better in light conditions and also work when motoring, which most wind vanes do not. Most can steer to a GPS track or compass course. However, they do use electricity and are prone to failure in the harsh saltwater environment of a bluewater passage.
I hope that helps a bit.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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