Marc,
Much of the "Features" become a matter of personal prefs. Also, you need to decide what you want in a computer and what kind of diving you do can make a difference.
Do you want a Full console computer or just a modular or "pop in" computer. First you need to figure what you want it to do for you and then find the computer that has the features you want. Dont be too worried about getting a computer just because it speaks, fetches the sunday newspaper does your horiscope, but rather does the job YOU want it to do.
All too often I see people get a dive computer because "it sounded cool" or it has more bells and whistles than the cockpit of a F-14 Jet. Sadly enough the "cool" wears off and you are left with something that will be outdone in the next model.
I mention this because you will all too often find some sales people that will try to convince you that you need all these fancy gizmos when all they really need is a sale. One of the best ways to get to know what you may like is ask other divers while you are diving to tell you or show you how theirs works and ask if they like it or if they would change anything about it if they could.
=-)