A few more notes:
Sniping is a pot luck with the first rifle you get. High center mass seems to be a instakill but I've had plenty of hit markers. There are a few maps with high vantage points, and you can close off the ladder access via a hatch, but none of your teammates will beable to get to it as well, and if you die, then that roof is no longer accessible. I was assault one round and my whole squad was sniping from the roof, I ended up with 3 defib ribbons with how many revives I performed.
As recon, you can't unlock all the sniper rifles by using a carbine or shotgun... you'll only progress unlocks through that weapon type as you use it. (Crap... I have to go through all that to get my GOL Magnum arguably the best sniper rifles in BFBC2). Every class has access to, Carbines, DMR (designated marksmen rifle), and shotguns when they are unlocked. LMG, Sniper Rifle, Assault Rifle, and PDW are exclusive to their respective class.
I did my first counter knife yesterday... its as easy as pressing one button. I'm not sure if its O or whatever you have as your melee button all the time or it prompts a random button. You have ample time to hit it and there is no point to knife from the front, because unless the other player just completely sucks, you'll lose every time... which is funny because my first successful knife was from the front. When a teammate is knifing someone from the front or the back, the enemy player becomes invincible so you cant steal their kill... however its wise to keep your cross hair on them on frontal knifing for the reasons I just listed. If an enemy is the knifer you can take them out before the process is complete.
Field upgrades are similar to squad perks in later ranks on BF3, except there are in sets now and you can gain each tier as your squad progresses... once everyone in the squad dies the field upgrade resets to zero. I don't think people on PS4 realize how important it is when "LAST MAN IN SQUAD" pops up on your screen... its even more crucial that you don't die so someone can spawn and the field upgrades stay intact.