I hate to add to all speculation threads, but I thought I'd just make 1 and only 1 for giggles.
I've played FPSs for awhile: half life->counterstrike->halo2->cod4->almost every other fps that died off almost right away.
I still go back to cod4, mainly because of the skill level ceiling is so high. Not quickscoping or even fast aiming "skills". But rather knowing the maps in-and-out, how to move, when to run, when to be cautious, when to be the bait and run across to draw fire while your team mows them down. 1000 little techniques like that. The way the match is progressing determined my play style as well, which I find most players don't adjust to. I was gonna make this a stupidly long post and give way to many examples so I'll try to keep it short.
Basicly, I started going back to starcraft, then sc2 when it was released. I've been playing it non-stop since release, because I enjoy the feeling of getting better and better and making improvements. I had dropped all fps games, and thought them silly compared to the skill ceiling of sc's.
Brink seems to be the bridge from cod4 for me (I know you all have your own FPS game that you love like me) to now that will now blow the top off how well you can play this game the more you play it. Just 2 friends who play together a lot and understand the game, will be a thing of awe, let alone an entire team. I miss the prime cod4 days where lobbies would stay the same for hours, and rivalries would organicly happen and force you to play better the next match to one-up them. Ah the good ol days.
In a nutshell, this game will rock and I can't wait