After about 30 hours of multi-tabling and 6000 hands, here's what I've got, in approximate order of importance:
option to turn off animation (slows down whole computer, can't act at any table while cards are being dealt at one)...nearly impossible to play more than 4 tables at once (my CPU usage jumps from 0-50% when cards are being dealt)
re-sizeable tables (be able to shrink down to 480x360)
fix "force to front" when it's your action on another table (put the actions for each table in a queue so the most urgent one pops to front, but only as soon as you've finished acting on your current table). Also, don't "force to front" when your active window is not a UP window (I've been typing things in other programs and hit "enter" only to find I've sent a random partial sentence to my UP table mates).
player notes
auto-rebuy in cash games (so you don't miss a hand - or worse, the button - after losing an all-in)
if we get disconnected, don't shut down the client! (hence preventing a return to any tables we're on)
automatically re-populate tables you're seated at when re-opening client (whether client was closed by a crash, disconnection, or even just manually closed for whatever reason)
"wait for big blind" checked as default when sitting down at table
bet-making with slider is not handy (clicking the bar should by default increase wager by only 1 tick)
"fold any" check box
lobby indicator of tables you're sitting at
waiting lists
better filters for ring games (limit vs no limit, 6-max vs 9-max)
min and max buy-in radio buttons (a "remember as default" option would be nice too)
when observer chat is turned off, a sound is still played when an observer says something