The best poker software I ever played on was on the PokerSpot website (now defunct).

Most important characteristics: Speed, non-cluttered visually, instant responsiveness.

In terms of responsiveness you pretty much want PokerStars-like speed, now.

In terms of visuals you want something like the original PokerSpot or a perhaps a modified UltimateBet type screen.

Forget the fancy stuff, just make something that works fast and smooth and is easy on the eyes. And offer your customers a choice of visual skins in various muted colors, and one in mostly black, but make almost all of them uncluttered.

Also, it can be annoying to have to wait for a hand to finish when you are not in it (at some sites) before being able to close that table AND take your cash balance to another table.

Make the lists quickly responsive and bug-free.

The faster the better for everything, really.