My advice: If you are going to design poker software, first of all you need to have someone who plays poker work with you help, if you do not yourself play already.
That said, my criteria, in order of preference is:
1.) Activity - Party/Empire is the king here with 15K + users on at any given time. (
www.pokerpulse.com for industry stats)
2.) Weak/New Players - Pacific poker is the king here, with some of the wildest games I've ever seen (5/10 80% view flop percentage is not unheard of) I think part of the reason for this is their regular "Showdown Survivor" promotion.
3.) Efficient Deposits/Cashouts - Although I loved playing at pacific poker, I stopped due to their 1 week withdrawal policy. Inexcusable
4.) Hand history Importable into Poker Tracker II software. (great stuff if you want to improve your game.
www.pokertracker.com)
5.) Comps and Promotions - Give me player points that I can use to purchase logo'd products with and provide you with free advertising. UB's Player point system is pretty good. Tourneys with and for player points is also a great feature. Offer deposit bonuses regularly. I, personally, don't get too excited about freerolls, but they are a great vehicle for attracting players.
6.) Software quality - This really is the last thing I worry about. My preference would be a mishmash of the following:
Game Selection/Lobby - Most companies are fine here. Party Poker displays insufficient game information in the lobby. Party Poker does have a VERY good game/player search function though
Statistics - The best user statistics display I've ever seen is on the downloadable client for pokerroom.com
Hand History - Should be easy to view and retrieve. Party Poker's is pretty good
Game variety - You need at a minimum: Hold 'em, Omaha, Omaha/8b, 7-stud, 7-stud/8b. Anything else is gravy, but would differentiate your software
Tournaments - Multi-table and sit and go's. Limit/Pot-Limit/No-Limit for all games that you offer. Pokerstars.com runs these very well, and has a great tourney information screen.
Private tables - Definitely allow players to create private tables, and private tournaments. Nothing like your having own poker tourney with old friends from college, now scattered across the globe.
Graphics and Sound - There's a lot of good places to get ideas from. True Poker has very innovative graphics, fulltiltpoker is pretty good too. Not a huge thing to me, but it doesn't hurt. Most places do adequate on sound. Just make sure you hear chips click and cards shuffle and dealt, not sure what you could do that was special here. Built-in microphone support or mp3 player maybe, perhaps an integrated internet radio?
Multi table support - You should support play at at least 4 tables
General Quality - I take lack of bugs, security, reliable Client/Server communication, and anti-cheating/disconnect/collusion algorhthms for granted. If these are a problem, you aren't going to get off the ground.
A final thought: You should seriously consider developing the software in java. There are very few poker rooms available to mac and linux users, and I imagine that these poker afficianados are screaming for more options.