If we are talking live aboard on the move and not parked in the one spot all the time, my priorities go something like this:-
1. Comfortable separate sea berth accommodations for minimum three people.
Why? While most of the time it may be only two people living aboard, when doing longer passages having three hands aboard makes the work a lot easier and more importantly SAFER. Each crew member MUST have their own bunk in which he/she feels comfortable and is able to get their required sleep/rest time.
Forget about SEX at sea. That is what you do on the beach in the moon light.
2. It naturally follows that the next requirement for the yacht is the water and stores carrying ability to sustain three people for 30 days.
Why? Even though anticipated passages may never be more than 14 days in duration, when one, or three in this case set to sea they are putting their lives in the hands of Neptune who can and often is a very hard taskmaster. He could decide to put the foolhardy to test to see how hardy they are.
3. Speed capability.
Why? As Joe Adams said, the faster a yacht is, the safer it is, and a number of other reasons. Slow boats are boring, fast boats are exciting. Fast boats get into the anchorage early for sundown, sundowners and BBQs while the slow boats are still out there trying to bash their way in darkness.
The above criteria obviously include seaworthiness and are all possible on a 30 footer such as a Spencer 30 or with a bit more style on a Lotus 9.2 as I have.