Here are the 7 deadly sins which can be committed by an online PC FPS title. Heed this warning carefully, FPS titles which commit these sins do suffer a painful fate (in the form of a quickly diminishing playerbase).
1. Thou shalt not create a server browser that lacks basic functionalities and features such as a favourites list, or the ability to even stop the browser from refreshing.
Explanation: No-one wants a buggy, badly designed server browser in an online FPS. The lively-hood of the game depends on an friendly and easy-to-use server browser. The more complete and additional features (friend's list etc) you can add on to extend past basic functionality - the better.
2. Thou shalt not ignore holding a beta-test for the purposes of bug-killing and balancing. In addition, thou should listen to thy feedback from testers and try thou best to implement into the retail game.
Explanation: For an online FPS to be fun and potentially competitive, it needs to be free of game-killing or down-right annoying bugs. In addition, it helps that the weapons and vehicles (if any) are fair and balanced.
3. Thou shalt not decide to release a dedicated server package 2 or more months after the retail release.
Explanation: This one is obvious enough. If you want people to be playing your game online, give out a free dedicated server package (preferably before the retail release) that lets hosts do their thing and provide servers for the masses.
4. Thou shalt not decide to not include joystick support if your title features flyable vehicles.
Explanation: 'nough said. Joysticks have been a supported PC peripheral for years, and for good reason. Ignoring this heritage is just not cool.
5. Thou shalt not decide to ignore support for widescreen.
Explanation: A personal annoyance of mine (owning a beautiful Samsung 226BW 22" widescreen monitor myself). When I say support widescreen, I mean all aspects (including a non-stretched hud or other anomalies).
6. Thou shalt not decide to shaft PC users over console users if the title is multi-platform.
Explanation: PC gamers who paid for their right to play the game deserve as much support as their console bretheren. This includes feature additions and map releases (which should be given for free to both sides).
7. Thou shalt not put patches through a flawed / unnessary long QA process.
Explanation: The QA process has a significant effect on the overall result and length of patch releases. If this is flawed in any way, bugged patch releases will destroy player-confidence in the development studio and publisher. In addition, unnessary long QA processes are bad if a pressing and considerably bad issue is currently occuring in the game.
Well thats it. I could of probably named some more sins but I'm lazy so I just did 7. Can you name any titles that would have fallen under one of the 7 sins I mentioned? Can you name another sin I didn't get around to mentioning?