Further to what's already been said, engineer class is by far the most fun for me, followed by recon. Here are a few tips. Apologies if some of this has been mentioned, I haven't read through everything.
Engineer:
Tank mines and the anti air launcher. MX4 or whatever it's called. Stick to close range, that default gun is a monster. Tank mines in choke points, and shoot anything that flies (choppers preferred). Be warned of scout choppers with repairbitches, you won't be able to get them down yourself. If a chopper has repairbitches in it and you get a hit on it, it'll be back at 100% health before you can blink. Unless you co-ordinate a proper attack with some others, forget abot them and ignore them. They're a known pain in the ass and it a constant point of contention on the BF4 forums as they're frustrating as fuck to deal with.
The launchers come ina a few different flavours. The first one has a small bit of lock on, but this is more of a guide than an actual lock. Great against ground vehicles, forget about trying to hit anything that flies with them. The next two launchers are much more powerful, each has a bit of a different perk though. They both only fire at flying targets unless a recon has a lock on a tank or whatever. The first will lock on, you fire, and that's that. It has a lock on range of 300m. The second one you have to maintain lock. So lock on, fire, and stay locked on until you hit, or the missile will stray off. The advantage of this one is either 50 or 100 mtrs more range, can't remember. I prefer the fire & forget one as I prefer not to stand in the heat of battle like a tit whilst I nurse a rocket to a chopper. Vehicles have ridiculously long flare reload times now, I think it's 15 seconds or something stupid like that. Use this to your advantage. Even if you don't have ammo as an engineer, forcing vehicles to pop their flares helps your team out, as someone else can lock on and pop their rockets.
I can't remember the name of the rocket launchers, I'm sure someone will add them.
I never really bother with repairing as an engineer unless working with someone. Odds are you'll start repairing a tank or whatever and the driver will bugger off halfway through, or if your'e repairing your own tank, it's not uncommon to have some prat jump in it and nick it. I prefer to play in a way that makes me as effective as I can be whilst being solo. Relying on team mates in this game will make you want to claw your eyeballs out.
Recon:
Recon is also great fun. Make sure you spot everything that you see, and also lock on using the zoomynightvisiony thing. Even if you're on a team that is all over the place, lock on a few times and see if any rockets appear. If they do, you know you've got an engineer somewhere on the team who's willing to fire at locks so keep locking. If not, then forget about it. I personally don't bother centering my scope. I am fairly good at adjusting for bullet drop, especially at medium ranges. A bit of practice here. I find zeroing my scope a fiddly process which leaves me vulnerable to attack whilst I'm sorting it all out, and I have to keep adjusting for encounters at different distances. I just aim a bit above their head an shoot. With enough practice, you will get better at this. One line of sight I will share which is simply awesome is on Flood Zone. Odds are you've always got those idiots who sit on the roof tops and try snipe people. If you hold down D (the petrol station) then you can face the buildings from the station and hide fairly well. Nothing beats shooting those dumbasses off those buildings and watching their corpses fall 50 floors to the ground. NEVER CAMP ON ROOFTOPS / CRANES / SATELLITES / WHATEVER. Seriously. This is one thing I have never understood with these people. They're either extremely hopeful or just plain fucking stupid because every match they end up there as if to say "AMAGAD LOOK GUISE I FINDED THIS OSSUM PLACE WHERE WE CN SHOOT PPL LOL". Yeah, and the whole fucking battlefield can shoot you too, dumbass. You're not the first person to go up there, you're not discovering anything new, you're just being an idiot and your KDR usually reflects this. Personally, I get a huge kick out of shooting them. More often than not, you can set your watch to them being up there. Time and time again. The rage I've gotten from shooting the same people off the same spot over and over again is bliss.
Of course recon now have C4 too. giggle giggle. This is great fun. Most tank drivers are aware of this though so you need to be extra careful when trying to blow them up. My approach is to take them out when they're pre-occupied with something else. If a tank is busy shooting at something, odds are he's in first person mode and focussed on whatever it is he's shooting at. This is when you strike. Also, NEVER shoot anything that's not silenced when near a tank as odds are they'll notice you and kill you, or at least move away so you can't C4 them. Basically, if you want to C4 a tank, you need to put on your stealth hat and only strike when he appears to be preoccupied with something else. It's risky but oh so fun.
I'll leave it at that for now, will return later.