This is really more of a personal gripe, but my thing is to never say a game sucks. You can say you hate it, you can say it's broken, but you can't say it sucks. That is simply too general for me, no game is so bad that it sucks on all levels and has absolutely no redeeming qualities. Especially when you consider that bad games are a great learning experience for designers. It's a lot easier to point out the flaws of a game, than the successes of a good one.
Also, you're not allowed to say anything about the quality of a game you haven't played. You can say it looks bad, or the game-play doesn't seem very fun, or all your friends say it's terrible, but if you haven't played it, you can't comment on how good is overall. This is especially true for all the MW3 vs. BF3 stuff going on, unless you've played both, you can't prefer one over the other.
