TANKS:
First off, pick the Tiger. It's the most underrated tank, largely due to the broken armor model. But when used properly, it can dominate. The key to it's use is to angle it properly: your enemy should ALWAYS be at 10:30 or 1:30. Always. The importance of this is magnified about 5 times because of non-sloped armor. If you're not angled, you will blow up in two T-34 shots. Though if you ARE angled, you can take 5 IS2 shots and live.
If that's not enough to make most people shun away, the awfully slow turret turning scares the rest off.
When these factors are combined, Tiger becomes the most difficult tank to master, but a real beast when you learn it. The key to the Tiger is driving. Unlike on most tanks, the driver has to both move the tank around AND AIM, because of the slow turret. Keeping the tank aligned against all targets when they shoot, and when the loading is finished, aiming at the right target, is difficult, but essential.
If you multi crew communication is essential. If you don't have voice comm, it's difficult. The best of you should always be the driver because it's the hardest job.
If you go solo, remember: angling your tank is more important than shooting. If you're facing multiple enemies, angle your tank against whoever you see first and shoot. His shot will bounce off due to your angling. When the next enemy pops out, pause loading and angle against him. When he has shot, angle against the first enemy and then continue reloading. Always angle against who will shoot at you next, even at the cost of slower reloading.
Do this and you WILL dominate.
Secondly, choose the tank that suits the situation best. Heavy tanks (Tiger, Panther, IS-2) are best used over long ranges and against tanks. From the infantryman's perspective, a light tank is just as dangerous as a heavy tank. Therefore, save the best tanks for tank vs tank action. Additionally as the soviets, the T-34 is actually BETTER against infantry than the IS-2 because of the extra machinegun position.
General tank tactics:
ALWAYS, I say again, ALWAYS MULTI CREW YOUR TANK!! A multi crew tank = 3 solo tanks of same class. You can dodge shots and perfectly angle yourself against multiple enemies, while reloading at the same time. The only situation when you shouldn't multi crew is when nobody will do so with you (everyone hops in a tank and drives off alone and if you get in their's they bail out and get a new one) which unfortunately happens way too often. People simply haven't understood the difference a skilled driver makes - it makes ALL the difference. It only really helps to multicrew a tank with 2 people, though, with the exception of some combined arms maps. The secondary machinegun is the driver's second position (and a welcome chance to rack up some kills as the driver), though driving is 10 times as important as gunning with the MG.
I say this once more: Drivers are underrated!
Anyone can point the turret at a target and shoot and hit, but it takes more skill to angle yourself properly against 3 tanks shooting at you from different angles while stopping whenever the gunner is loaded, and keeping the sappers at bay, etc...
If you have a heavy tank, stay back and dominate open areas, preventing enemy tanks from advancing. If you have a light tank, support infantry when they capture objectives, or when facing tanks, always attempt to flank them or close with them. A light tank will be heavily disadvantaged when fighting a heavier tank over range. The closer you are, and the worse he is angled against you, the smaller the disadvantage comes. Up close and from a flank, a light tank can destroy a heavy tank almost as easily as a heavy tank could.
(German) If you face a tank from the front, aim at the driver's hatch. It appears to be a weaker spot. From the side, target ammo location under where the front of the turret touches the hull. This is the only way to get a 1 shot kill. If you're not confident, aim at the engine (rear hull) - it's a much larger target and almost a guaranteed 2 shot kill. From the rear, just aim in the middle. You will get only one suprise shot - it must hit. Besides, the engine location is right there, plus even a light tank shot from the back will be devastating against ANY target.
(Russian) Same as above really, except the ammo locations are a bit differently stored. Look at the data sheets. A rule of thumb against the german tanks is to aim at the least sloped part possible, if ammo locations / engine aren't easy targets. This appilies to a lesser extent as German against the Russians, because generally the Russian tanks are a lot better sloped.
INFANTRY:
Pick the rifleman - voluntarily. It's the most underrated class, yet it's the best overall class aswell. He dominates medium and long ranges and together with the sniper, can even pop out and kill machinegunners. In close quarters he is better than you'd think. Yes, the SMG gunners are better. However, one shot from the bolt actions kills nearly every time. Therefore, use the iron sights a lot even in extreme close combat - if you hit the SMG gunner first, he will die, and even if he hits you at the same time, he'll die and you have a good chance of living because the SMG's aren't powerful. But if you miss, you're dead. Against a rifleman, though, it's even odds and a bayonet can help. Absolutely no other class is as verstale as the rifleman.
As the sniper, hold back a little. If you haven't noticed, at medium ranges, though slightly more accurate, a sniper is really only as powerful as a normal rifleman. To make the most of it, hang further back, out of the reach of submachinegunners and grenades.
The same rule of thumb applies to the machinegunner.
Points (out of 10)
Rifleman:
CQC: 6
Medium range: 7
Long range: 8
SMG Gunners:
CQC: 10
Medium range: 4
Long range: 2
Semi-auto rifles:
CQC: 7,5
Medium range: 8
Long range: 6
Machinegunners:
CQC: 3 (pistol / less mobile than the sniper if deployed)
Medium range: 10
Long range: 9
Snipers:
CQC: 4 (pistol)
Medium range: 8
Long range: 10
STG 44: (the use of this IS different from the SMG - you can effectively engage targets at medium and long ranges, unlike with the SMG's.)
CQC: 9
Medium range: 6
Long range: 5