Okay, I've got 17 or so hours logged on multiplayer so far, I'm about level 43.
Right, regarding Guns, it's clear that Assault Rifles are the dominating class, and for good reason. The FAMAS and Scar-H make incredible low-level guns, to the point that I get fairly irritated with the amount of SCAR's around, but who's to blame the users, when you are a low level, there's really not that much else to use. I also unlocked the M16A4 and I'm sorry to say that it really has not changed a bit. It's still as powerful as it was in COD4, and it's gonna be really annoying when everyone hits level 40, for sure.
As for SMGs, I really don't like any of them other than the P90, which while it has been downgraded a little, is still awesome, to the point that I have Blue Tiger on it, and am about 17 bullet penetration kills from Mastery. Mini Uzi next level might be alright, who knows.
LMGs are still good, but I never really use them, so I can't say much. One of my nub friends pretty much whores the RPD though, and he seems to do pretty well.
Snipers are sweet, obv the Barrett is incredible as it is the balanced all-round Sniper most people are used to, and works well with pretty much any attachment. However, the ACOG on the Intervention and the WA2000, which are the only two other snipers I have unlocked, is incredible. The Intervention works as the slow firing, one hit kill sniper and the WA2000 the quick firing, two hit kill, but both barely recoil at all with an ACOG, and I've gotten a couple a triples so far with I think one triple from both of the ACOG snipers.
Riot Shield works wonders if you have a teammate stick with you, and even then only really on maps like Terminal with narrow corridors. SEMTEX grenades go well with them I've found, mainly 'cause a lot of people switch to Riot Shield themselves when I am using it, meaning I can just stick them and run away ^_________^
I'll talk about Perks when I get them all Pro and Killstreaks when I unlock them all.