I think my main problem is that I have not really settled on what kind of gun / perk combo I want to use. Some american player criticised my choice of gun in one of the maps but the only way to upgrade the weapons is to use them.
I'm using the M14 just now - yes it's a one shot weapon but with double tap and steady aim it usually kills with two shots (without aiming down the sight) and once I get a scope for it it might be handy for mid range sniping. I'm also working on my G36c challenges.
As for perks I have an 'offensive assault' setting (m16, 3 frags, stopping power and the extreme conditioning) and a 'defensive mode' (m16, claymores, stopping power, deep impact).
I have a 'run and gun' setup with p90, bandolier (extra ammo), double tap, extreme conditioning for house to house or more compact maps. Quite a rate of fire!
I also have a 'walking bomb' setup (p90, 3frags, more explosive damage, martyrdom) - I try and avoid using that on hardcore modes.
My 'ghost/stealth' setup is silenced M16, claymores, UAV jammer, and extreme conditioning. I was using 'dead silence' but I think the ability to get to the objective (or access points to it) quickly and lay a couple of claymores is more advantageous than the occasional sneaky knife kill.
I suppose weapons and perks are all down to personal preference. We could probably start a whole new thread on the various combinations. I think what I am finding is that even though I have my basic setups above I am constantly tweaking them from map to map (sometimes for the better - usually for the worse!)