World of Tanks is a strange beast when it comes to balance -- essentially there isn't any. There's no attempt to make things fair for individual players, instead "balance" is acheived by having the teams randomized and roughly balanced against each other by the tier ranking of the vehicles within each team. One Tier 2 medium tank might be signficantly more powerful than another Tier 2 medium tank, but unless one team gets all of one type and the other team gets all of the other, it doesn't really matter. Also, generally speaking, there's always someone on the map in a weaker vehicle than you, so if you can't beat anyone else, go find them.
Which leads to some rather unfortunate . . . quirks. Progression in World of Tank consists of driving a low tier crappy tank until you earn enough xp to research the next level of tank, then switch into one of those and drive it around until you earn enough xp to research the next level, and so on. There are ten tiers in the game, with a tier 10 heavy tank roughly twenty times as powerful as a tier 1 light tank. You start out in a tier 1, battling against tier 1 and the occasional tier 2 tank. Once you get into your own tier 2 tank, you battle mostly tier 2 tanks with the occasional tier 1 tank. It's a pretty fun time -- most enemies are either roughly equal to your or weaker, giving you good odds of performing well and having a fun time. So far, so good, right? Then you upgrade to tier 3 . . . and suddenly most of your battles consist of tier 3-5 vehicles. Despite spending all this time and money to get into a tier 3 tank you're suddenly cannon fodder, and every enemy tier 4 and 5 tank is going to be gunning for Mr. Easykill.
I think this is World of Tanks' hazing ritual. You're getting into the real game, the training wheels are off, now prepare to be spanked. Repeatedly. It's a bit of a shock. Sort of like showing up to a frat party blindfolded expecting to party with a bunch of sorority girls, only to have the blindfold whipped off and find a bunch of football players standing around you with paddles and wicked grins on their faces. (Note: I was never in a frat and never went to frat parties :p)
My experience so far is that most matches have been about 30% tier 5 tanks, 50-60% tier 4 tanks, and the remainder are tier 3 tanks. To put it in perspective, a tier 5 tank has roughly double the hit points of a tier 3, between half again and twice as much armor, and a gun that's roughly twice as powerful. They are also faster and more maneuverable. A tier 5 medium tank can one-shot my tier 3 medium tank with just a little luck. The tier 4 tanks aren't so bad, but fighting tier 5s as a tier 3 is insane. It would be roughly equivalent to battling a level 40 in World of Warcraft as a level 20. It's not very pleasant, and I haven't really played the game in a few days as a result.
However, logically I know that this can be worked through. It really is like a hazing ritual, since all those big boys in the tier 5 tanks gleefully one-shotting me were once in a tier 3 getting their metal avatars blown to bits too. Even if you get destroyed, your team could win and you'll get solid xp and credit rewards. Even if you lose, you get something. Clearly I can get through tier 3 eventually and join the more balanced ranks of the tier 4 tanks, but it's going to be something of a chore. Also, being in a tier 3 doesn't mean never getting kills in these matches. Taking out a tier 4 is possible, and of course other tier 3s are fair game. Plus, self-propelled guns don't have much armor even at tier 5, so they can be taken out too (as long as they don't notice you, of course). Once you're spotted by a tier 5 though, it's all over unless someone on your team takes them out while the enemy has tunnel vision gunning for the easy tier 3 kill.