These are just my observations from looking at boats and doing some reading on what generates a Cat A bluewater boat.
1. Hull structure - How the hull is made to take the bending and flexing movement. Bonded bulkheads, stringers and interior pieces to the hull create a stronger hull.
2. Hardware - beefier and attachment points stronger
3. Ports - No plastic here. Ocean waves will create failure of most plastic ports.
3. Cleats - Be strong enough to deploy a drogue or chute without being riped out by the forces.
4. Anchor platform - Beefier to stay on the hook during a blow without bending or failure
5. Stability Curves - Boat be able to right itself in-case of a knockdown over 110 degrees
6. Hull Shape - Be able to ride out rough seas and still be in control of the boat, Ease of motion helps here too. No long overhangs to keep following seas out of cockpit.
7. Rudder attachment points - at least two(top and bottom) - three is better.
8. Deck - being able to clear water over deck fast
9. Cockpit - small to medium with ability to clear water out fast.
10. Liferaft storage to be deploy easily
11. Rigging and chainplates - beefer and oversized. Chainplate attachment points to hull vs bulkhead with beefer stringers to spread the load.
12. Fuel and water tanks - below waterline for more stability. Also in my humble opinion min of 100 gals fuel, 150 gals water for 40' or less boat.
13. Quality of construction of boat.
Just a few I can think of this early in the morning until 2nd cup of coffee