BIG, BRIGHT NUMBERS!!! I'm a member of the geriatric set, and anything small is unreadable underwater, even with the bifocals in my mask. I love not having to fuss with a backlight or getting precisely the correct angle from my dive light to read numbers in the dark. Bright displays rule!
Beyond that, I like a fairly small unit, because it's on my wrist. An intuitive interface is critical, because if the navigation through the menus is too complex, I can't remember it underwater. I liked my Tec 2G a lot, but hated the fact that I could never remember which button you pressed and how long you pressed it. Downloadable is non-negotiable, but since I invested in DivingLog.de, I no longer worry about whether the software that comes with the computer is any good or not. (I thought the Uwatec software was fabulous.). Customer service is also important, because computers DO malfunction. Liquivision has been very good to us with service, but the support for our Suuntos has been spotty.