I just thought it would be a good idea to add a topic that people share knowledge with others that may be helpful.
These are the things people have posted so far:
pepsiman3000- you can set a distance with the XM-25. It will explode in mid launch at that distance unless you set it differently or die
ShadySLAYER666- If you are using any equipment and decide not to place it or throw it, press Y and it puts it away (remember this doesn't work with frag grenades, emp, concussion, or flash).
pepsiman3000- Recon Drone has two sets of flares. It only takes ~3 shots to destroy though(AR or SMG).
HOOShenry- Also, on some of your secondary weapons, (I think the FMG9) you do a little gun flip that takes up time, but this only happens the first time you switch. Therefore if you do it at the beginning, you can switch weapons faster later on when you need to. (On some secondary weapons, the first time you switch weapons, the initial animation takes extra time to switch)
OChooligan- When using the javelin once you get a lock on target you can move your character any direction as long as you keep the lock (the white cross) in the middle of the javelin window. this can be used to give yourself enough frontal clearance to launch the javelin without killing yourself after obtaining a lock from a position that you would normally be unable to fire from.
MrStrad- For people who don't already know this: the best way to get a good killstreak going or a run for a MOAB is to run back and forth throughout the outskirts of the map, use specialist with perks that would help you the most, use a weapon that you are extremely comfortable with, be very cautious, and don't give up.
HOOShenry- When you are planting or defusing a bomb, you can still turn from side to side. This is helpful so that you either know where an enemy is to kill once you are done, or worst case scenario, you have to quit to kill them.
pandamancan2- double tapping x will set off the c4 for those of you who didn't know
You can drop/crouch around a bouncing betty, best choice is to drop to prone though.
when facing a jug. 5 knives kill them, direct impact with a rocket. Also a combination of shooting then knifing seems to be the best to take jugs out. grenades don't do great damage too them either.
TheKaosWithin- You can use grenades while on ladders
IM U IR ID E IR- When using throwing knives I've found that having aim assist turned on helps tremendously. When someone is in front of you the aim assist will automatically sense them and pull the cross hair towards their body. Do not aim prior to seeing your target. You should just tap the RB once the cross hair starts to pull. Its helped me significantly.
OChooligan- You can save time reloading by doing the reload cancel which entails tapping the sprint button the second you see your ammunition replenished on your HUD
SoNiC HD TriCkZ- You can tag UAV's with Recon Drone.
pepsiman3000- You can start reloading special weapons and launchers faster if you manually reload.
MissPEWPEW- You can put portable radars/anything tossable in the crack between stuff (like dumpsters/crates against a wall or against each other, as long as they meet in a 90 degree corner). The enemy will never find them unless they have SitRep on.
ChrisBegley-The shotgun attachment ammo can be refilled if you drop the gun, and pick it back up.