Most of these are not game play mechanic related, but could really help improve the overall experience of the game.
When an objective changes based on the situation context, like when the NPC escort goes down, the objective should automatically change to the new objective if a player has that objective selected. I.E. if Nycheyev goes down and it switches from “escort” to “guard” it should just change automatically instead of cancelling altogether.
On that same note, I don’t even think that the objective should change names at all, except for the pertinent classes. Using the same example as above, everyone but medics should see “escort” while medics would get (and if “escort” was their selected objective, it would switch automatically to) “revive/heal.” The fact that it says “Guard” when you’re supposed to kill the escort on some missions is EXTREMELY confusing at times.
Still more on objectives and being completed, if an important mission is completed, but it’s not one that plays a cutscene, an audio clip and a hud element should inform everyone that the objective was completed. I see too many people, and have fallen victim of it myself, going to the WRONG objective (such as going to the safe when you already got the data key out) because nothing informed you that the objective was completed.
The perk… er… “‘Whatever’ of Perception,” would be a great perk to have… If it weren’t for the fact that without it, you can actually move around and take cover while doing an objective. Maybe make it so that if you don’t have the perk, you’re focused on starring at the objective, as well as being unable to move/crouch/lean.
Likewise, with Perception, it would be nice if it worked for everything, and not just repairing/building/planting. Disguising, comm hacking, hacking turrets, repairing turrets, etc all do not benefit from the skill, which doesn’t make a lick of sense.
A visual indication of what a spawn room is (or better yet, make them exit only) would be nice. I’m always ending up finding the enemy spawn and greeted by their death cannons. Yeah, not so much of a problem once you know the maps inside and out, but think of the new players later on.
Customizable crosshairs. 'Nuff said. 
Server Browser Filters that aren’t stupid. Some of you may have seen my posts asking how to work them, well I found out, and it’s pretty dumb. When you open a filter, you’re presented with an input box. The input box only allows a single number to be entered, and no mathematical or programming syntax seems to do anything. So, if I put “12” into “players,” the browser filters out every game that has 13 or more players. For ping, this is ok; put in 150 and you get only servers that ping up to 150. But for player counts, this is dumb. Where’s the “not full & not empty” option?
Hitbeeps. The cvar is there, but it doesn’t do anything. At least not in MP. There are some commands, such as this and “static crosshairs” that only seem to work in Solo/Challenge mode. I remember hearing a quote from SD about this, saying, “We made sure that when you hit an enemy, you know it.” Having the XP counter and a red thing over your crosshair is ok… But I’d still like the hitbeeps, and so would many other people. If the option is present, and works in SP, just make it work in MP too.
As above, but for the static crosshairs. Make 'em work in MP.
An option to disable depth of field or a fix for it. I think it’s actually working in reverse. DOF in MOST games happens when you ADS. DOF in Brink DISAPPEARS when ADS, but is annoyingly present when not looking down the sights.
Something that tells you that you won’t get unlocks in Challenges if not in Solo mode. Unless this is a bug, of course.