There are a lot of things here and there I'd like to see changed or included or what have you, but playing this series there are two things (specifically from playing BC2 recently) that I'd really like to see.
The first is multi-squad support. I've got plenty of friends who play, and it's always such a pain to try to first get us all on the same side, in the same room, and then try to play together when we can't see what half of the party is doing. I don't think it's unreasonable to have support for up to four squads of up to a total of eight players to be linked together, to load into a match together, and to be able to see one another's tags on the radar/onscreen.
Second is the way players react to damage. It's incredibly frustrating to get several solid center of mass shots only to then be killed by that player and find their health at 2% (or 0%! I've seen it happen a couple times), with them running around like nothing's wrong.
Now, I'm not looking to start a realism argument, or to discuss where that line lays, but I think it would add a little extra immersion, tension, and impactfulness to the gameplay if when a player's health is dropped to less than ten percent in under three seconds, he would stumble a little, with blurred vision and muffled sound. Then if he survived he could get back up and keep on trucking at full athleticism.
The number one most frustrating thing I find, most often, is that. I've been shot before, and let me tell you, if you get shot, you go down. I'm all for getting back up again, and I'm not trying to suggest extra realism purely for realism's sake -- instead, I think it would improve balance, decrease "cheap" feeling moments, and add an extra bit of dynamism to the game (two players exchange rounds and both stumble, who can recover faster?), as well as some fun set-pieces when all the explosions are going off.
Perhaps they could even add a feature where you fall to the ground after really bad damage (big falls, explosions, collapsing buildings, going from full health to a kitten's sneeze away from death), but you get back up. Balanced in its own way because if players don't confirm their kills the wounded man could recover and pull away the win, but taking the time to confirm could leave one vulnerable.
No, I mean this is entirely crack cocaine. All of it. Just... just so much crack cocaine, Robert. You've been doing this all week? -- Jack
in BC2, in hardcore mode, when you're health is really low, the screen goes red and it's REALLY hard to see anything. It doesn't recover either, unless a medic heals you. It's been a while, but I'm pretty sure the sounds muffle as well.