Fit is a must.
If you decide on an integrated weight system, I would say it is a must to avoid any where the pockets are held in only by velcro. They have a bad habit of wearing out and dropping when you don't want them to - actually, I've seen it happen on brand new ones. Just a bad idea that I don't think too many manufacturers use anymore anyway.
It has to hold the tank securely, which some may do a little better than others, though I doubt there are any out there that are really unacceptable and problems are often user error. Some people prefer 2 straps for this reason, although Scubapros have one tank band that works really well
It should have enough lift for your diving. A travel oriented BC that doesn't have much lift may be perfect for someone who only travels, but inadequate for cold water diving (though most BCs will have plenty of lift for the average diver.)
Pockets, WI or not, inflation style, attachment points, all sorts of stuff like that all depends what you like and what you're doing. Unless you are talking about something like the Hub, which most people will agree is full of feature that are downright stupid.
Now if you are planning to get into any sort of technical diving, that's another story altogether...