Like others we have done a fair amount of live-aboards, from basic to luxury. Our deciding factors in order are....
1. How many people does the boat take. I prefer boats that take 16 or less. The less the better. 10 is about perfect.
1a. Length of trip. We love 10 days plenty of diving.
2. How many dives per day? Minimum of 4 with no cap on bottom time. If they do 5 great, open dive deck even better, solo diver program Fantastic!
2b. Will they let us dive our own pace. As amateur photogs we move very slowly.
3. Reviews and Reputation. A poorly maintained boat or unmotivated crew can ruin a trip. Plus good dive guides can be the difference from seeing pygmy seahorses or not.
4. Have to agree with the private bathroom. When you are doing 4-5 dives a day, eating, drinking I don't want to be in a queue for the loo.
(For us camera friendly boat is also important with a decent camera room, charging stations and camera friendly crew. Can be another reason rooms can be important if you are doing all your camera work in there)
In general you get what you pay for!