I got this idea from a gamefaqs thread that turned out to have some pretty useful info in it.  Still, the game does such a terrible job of telling you useful information that I think this thread would be helpful.  I'll start.
 
1. you don't need to spend skill points on resuscitation (the first one) if you get improved resuscitation.  
2. you can change weapons with someone who's dead, change back, and you'll have full ammo
3.  When you're a squad leader press R1 in the map to call in airstrikes (people don't know for some reason)
4. burnoff and cooling towers give you frago points by default, so there's no need to make it the squad objective (they should know to go there anyway).

-If you have a frago on your bunker bunker gets a charged placed on it  and you defuse it the frago goes back to the burnoff tower. Switch it back. 
 -At the beginning of the game if you can't switch loadouts hit start then O and you should now be able to switch loadouts. 
 
Only ones I can think of right now.

GET A MIC! 
 
I need one >.>

whoa those are actually all useful so far- im level 53 and i didnt know ANY of those, cheers!!

-You can resupply from the bunkers. I once kept seeing my squad run to a resupply point FROM the bunker.  
-www.maggametools.com <----talent calculators, loadout calculators and weapon stats. 

-You can also resupply by getting into the APC. 

If you can get into the APC. for some reason I always have trouble getting in since no prompt shows up. 
On SEVR maps the battered gates have a hole in the bottom that allows for totally safe disarming, much to the dismay of the attackers. 
Many maps have small gaps or windows that generally serve as great, nearly totally safe, sniping spots. Keep an eye out.

1. you don't need to spend skill points on resuscitation (the first one) if you get improved resuscitation.  

If you are unsure what your objective is hit the down on the d-pad. 

If you are squad leader, to assign objectives faster rather than hitting select, hit the down on the d-pad and find an objective prompted by icons and hit R1. This is much easier directing your squad on the go while stealing being able to see what is going on. 

If you see a fallen ally REVIVE! that is 10xp (and +10 frago if you are near an objective), and remember not to bleed out if you are shot in an area were people can get to you. 

If you are revived and find yourself stuck in an object (happened a few times) hit the start button and push R3 to kill yourself.