World of Tanks has a fun premise, but the execution is flawed and the developers do not listen to the community.
There is a fair amount of variety. You have your Tank Destroyers with weak armor and limited gun traverse but a strong gun to make up for it, light tanks with high speed and manuverability but low powered guns, medium tanks with a nice mix in the middle of power and speed, heavy tanks with slow speed but high armor and powerful guns, and artillery (SPG) which have expectedly poor armor and manuverability but are able to deal massive damage to tanks via indirect fire from afar. Matches tend to be a mix of these tank classes.
The game has detailed gun information and tank stats which try to emulate reality, but many times they are outrageously unrealistic. At times, a gun that can penetrate 203 mm of armor can bounce off of an artillery with 13 mm of armor doing no damage. These occurances can happen far too often in the game and ruin the gameplay. The larger problem stems with the balance of gold tanks vs. regular tanks. Regular tanks require XP and credits to unlock, premium tanks require gold to unlock (Gold being purchased with cash only). Most premium tanks are decent, but not game breakers by any means.
That is until recently, when the Type 59 Chinese tank was added (A tank from the Korean war). This tank is rated as a Tier 8 tank with the base line tanks being Tier 1 and top tier tanks being Tier 10. Tier 1-5 are relatively fast to get to, 6-7 are moderate, 8-10 require long grind times. There are Tier 8 heavy premium tanks in the game, but due to their slow speed they are easily countered. The Type 59 however is a speedy medium tank with armor that bounces many shots that hit it. The game's matchmaker system also frequently creates imbalanced matches which pit 4 Type 59's on one team against a team with no Type 59's. Battles tend not to spread more than Tier 5-7, 6-8, 7-9, etc. but the Type 59 regularly is pitted against Tier 5 tanks. Its high speed and powerful gun make it a game breaking tank for many players, but if you dare criticize it, the community just calls you a complainer despite the fact the tank DOES imbalance the game quite a lot and is a premium tank (Which makes it a "Pay to win" tank - a big problem with F2P games).
The development team sees no problem with the tank and has stated they will do nothing to change the tank's armor, tier, matchmaking, or anything - because it's just peachy as is. The game also loves to place 4 Type 59s on one team and only 1 or none on the other team making it harshly unbalanced. Matchmaker issues would rank as the most glaring problem with this game. There is no skill level system associated with a player, therefore it is possible to have teams of high tier tanks that are terrible and teams of low tier tanks that are excellent players. Many times the matchmaker results in insane defeat numbers. You are frequently placed into battles where you have minimal usefulness to the battle, so your choices are to attempt to fight and do minimal damage and likely die or hide and not assist the team which also doesn't help. Previously, using HE shells would damage any tank because of HE's explosion on the outside of a tank - but the recent 7.0 patch nerfed HE making this far less effective (Example, a standard HE gun mounted on an American tank would take a tank from 100% to 80-85% in one hit - now the same hit will take that tank to around 90-92%). This means you have almost no useful purpose in badly placed matches.
Frequently matchmaker will place more powerful tanks on one team and try to balance one powerful tank with two lower tier tanks - but it does this by giving the enemy team say 3 powerful tanks and the other team 6 less powerful tanks. Given how poor teamwork is in this game (Due to the lack of a skill system that can judge if a player is good or terrible at the game) that means typically one of the weaker tanks will attack the powerful tank. One by one the tanks are killed and the enemy team wins because their more powerful tanks even with no teamwork can destroy the other team's tanks easily. I've played this game for over 300+ Hours, it used to be a ton of fun. Sure everyone had an occasional loss streak, randomly matched into say 4-5 losing battles in a row but it wasn't real common. Now it's the norm. Most of my tanks that I am skilled with still end up losing mroe than they win.
The community is also rubbish. Any time you complain about game issues, others pull out the "Then just quit" card or the "It's free" card. This is a tired argument, just because a game is free does not mean it can escape criticism for its flaws. The game had its issues before, but the newest patch has raised more issues. The developers continue to keep working on things like adding horns for the tanks, new camo paintjobs for tanks (that do nothing but change your tank's appearance and cost gold for a permanent paintjob - which is what most people would want, not a week of camo), and the next patch v 7.1 is supposed to add an entire new country - France. So instead of balancing out the game before adding more tanks in, they are adding an entire new country which will be unbalanced at the start. In fact, the French tanks will have autoloaders so they will be capable of firing 5 shells in rapid succession and their guns are rated at high penetration, exceptionally above the tiers the tanks themselves are placed at. This is supposed to be countered by their very weak armor, but the amount of highly unbalanced gameplay that will exist for awhile should be quite obvious here.
The game used to be more fun than frustrating, but I find all I end up doing is spending hours playing through defeats in my tanks just to earn one victory and being irritated by the time I'm done with each of my tanks for the day. There's no doubt this game could be fixed, but the community tends towards insulting anyone who raises criticism of the game and the developers don't really listen to the community either - so player feedback is very ignored. I will be shelfing this game for awhile, as it's simply not that fun anymore. A few months ago I would have given the game a solid 7.5 or 8, but as of present with the 7.0 patch the game is lousy. The matchmaker needs to be balanced, the Type 59 needs to be more appropriately matched in the matchmaker, and a skill system needs to be implemented to rank players. As it is, victories feel more like your team isn't as bad as the other team - not as though your team is smarter than the other smart team. If this makes sense to anyone, this kind of teamwork is lousy. It might appear like teamwork, but it truly isn't and public matches need to be more teamwork oriented.
Update 3/14/12:
I have continued to play this game, in hopes it would improve - sadly it still hasn't. I would also like to draw attention to the fact the game suffers from excessive moderation. People are frequently chat banned for really frivial matters (I've seen someone chat banned for a day for calling someone else an idiot - and if you play this game you'll want to call people a lot worse than that). The one I find the most pathetic/comical is a rule saying you can't talk bad about the Moderators/GMs/Developers. Clearly they feel like their work is a masterpiece with no flaw, so they can't accept criticism. I've even seen the moderators chime in when people start to talk about another game with something like "Hey guys, lets keep the chat about tanks okay?" Honestly, I can't think of any MMO that cares what you talk about as long as you don't threaten or harass someone else. I've been chat banned twice for stupid things that no one would care about in any other MMO. Not only that, most games would just chat ban you then you just can't talk - no, in World of Tanks the moderators post that they just banned you in the chat. Apparently it's supposed to "scare" everyone else into submission. What a joke...
There's another problem I have since learned about. The game takes a long time to grind through, in 6 months of rather regular playing (I played to win on every tank I was "grinding" on that was non-elite almost every day - sometimes with x3 or x5 XP bonuses). I am now at tier 8, the game has 10 tiers. In my tier 8 tanks all I do is lose money no matter if I win or lose a match. XP gains are extremely low as well, so progress slows down even worse. I'm currently using the American T32 - a tank regarded by others as a "good" tank, I find on a victory I made less and actually lost money in this tank whereas a tier 5 tank I used on a decent defeat made 20k credits of profit and MORE XP. The devs lie and say damage determines your credit and XP gains but it's a load, they don't. The devs basically don't want you to play a high tier tank without wasting money on their stupid premium junk, I will not and have never paid them for this. The game ceases to be an even occasionally fun Free 2 Play at tier 8 and becomes a Pay 2 Win game (This tier encourages the purchase of overpriced premium tanks with gold that have ridiculous credit modifiers).
Most games would gradually increase the profit you make with the tanks as you go up the tiers along with their repair and ammo costs. Well World of Tanks follows part of that formula - the tanks cost WAY more to buy, WAY more to repair, WAY more to load up with ammo, but they follow a bell curve approach with tier 5 and 6 tanks at the top. As of this moment, Tier 1-4 makes no money so they're horrible grinds to unlock the next tank but they swear they're fixing this - and nerfing the higher tier tanks income further. Regarding the Type 59 I mentioned in this review earlier, they removed this premium and warned the users that they were doing so in a reverse psychology attempt to increase sales of the Type 59 before it was removed from the store (It worked). The people running Wargaming are amateurs and it shows in their back and forth attempts to make the player base think they're listening and that they care when they really don't. Sure, other game companies can be this way too - but it's obvious that Wargaming sees us as nothing but dollar signs. They ignore complaints of maps held by the majority of players that we all think are bad maps, yet they're in permanent rotation never to be removed or fixed in any way.
I'll advise anyone who wants to play this of one thing - do NOT waste your money on this game. Play, but don't spend a dollar - Wargaming is a really poor developer and deserves to know this by its monetary gain (Or rather lack thereof). Maybe some day they'll learn to listen to their playerbase instead of chatbanning them for criticizing the developers.