The biggest downside, in its current state at least, is that the mid tier tanks are a crucible of grinding and being essentially useless in matches- while the occasional bounced shell is not unheard of once you have better guns, a number of tanks will be invincible against you, for all intents and purposes. It's sort of balanced out by the fact that HE shells can be loaded, but even then, you'll run into tanks that you can only scratch a few percent off of their total health. This wouldn't be a problem if you could escape and let your more powerful allies step in, but often times you'll be in a situation where the enemy tank can not only see farther than you, but is also faster and more maneuverable, negating any chance of escape. Once you grind your way up to the higher tanks, though, it gets much more enjoyable. However, the downside for a losing match becomes much higher, as you can end up losing money.
I had originally planned to write a much larger article, but I'm holding off until the retail release, since a number of things (like artillery) have changed & there is a lot of discussion about gameplay mechanics going on, as well as planned expansions like French Tanks, et cetera...
Also a hard wipe is coming soon, which means I will lose all my glorious tanks:
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I think I'll withhold my judgment until retail is out.