An MMO site that doesn't mention World of Tanks. Kind of weird, time to fix that. Most recent thread was from early this year (well, nearly last year I guess.)
World of Tanks is a F2P MMO focused on, you guessed it, tank combat using tanks - both tanks that were in production as well as just prototypes and design stages - from the early 1930s to the early 1950s. Currently, Germany, USSR, America, and France are all fully-represented. Britain has a limited selection of tanks available, and China is slated to appear before too long.
There are ten tiers of tank, with each tear becoming progressively more powerful and expensive, and matchmaking is mostly done via tank tier to help ensure you're only going to be playing against tanks that will be appropriately challenging for your tank. Tank varieties include light tanks (mostly used for scouting and flanking), medium tanks (the Mario of the game), heavy tanks (very tough, very slow), tank destroyers (basically snipers) and self-propelled guns (SPGs; artillery so generally just referred to as arty.) Each nation has its own style of tank, and each individual tank is different. You can check the World of Tanks Wiki for more detailed information on each tank.
The game is surprisingly detailed and fairly realistic in a sense. Tanks are all more heavily armored on the front than on the sides, back, top, or underside, and armor slope and attack angle are extremely important. You can "bounce" (ricochet) shots off a lightly armored tank if you're attacking from a poor angle, and can blast right through even the rolling fortress Maus if you hit a 90 degree angle on its weaker armor sections.
This results in the game being a little bit unfriendly to newbies, and one of the most frequent complaints are about shooting a tank and doing no damage. This can be for a multitude of reasons, with armor slope being the most frequent - again, the farther way from a direct 90-degree angle you are, the less likely you are to penetrate their armor. On top of that, you can cause damage to modules (such as tracks, engine, ammo rack, radio, etc) or crew members (dead crew members mean the job they're responsible for takes much longer and is less effective) without causing damage to the actual tank. In fact, this is a pretty important part of playing at higher tiers and especially when you're playing a tank and can't penetrate an enemy tank's armor. Tracking a tank (destroying its tracks, rendering it immobile until the crew fixes them, which takes several seconds) leaves it a sitting duck for your team's arty and heavies and can often mean certain death for the enemy tank. Likewise, landing a good hit on the ammo rack can cause it to explode, causing enormous damage to the tank, and hits to the engine can destroy it (leaving the tank completely paralyzed until it's fixed) or cause it to catch fire, causing damage over time.
Admittedly, as a F2P game, World of Tanks is definitely pretty grindy. Buying gold (cash shop currency) allows you to buy premium account status, which increases the XP and credits (game currency) you earn from battles, but it's still pretty grindy. You can expect about 300-500 XP for a good game, and maybe double that for a really great game, and you get double XP for your first win of the day... but it still quickly becomes somewhat grindy, with tanks beginning to cost 15,000 and 25,000 and even more XP; many tier 10 tanks cost millions of XP. This is intentional, but it can still become annoying. Unfortunately, there are no XP or credit boosters you can buy (which are present in most other F2P games), so after you upgrade your account to premium status, that's about it.
Otherwise, not much of the game is play-to-win. You can spend gold on special ammo which is better than the basic rounds, but you can also buy those special rounds with credits. Gold can also be used to convert "Free Experience" (XP which can be used to unlock things for any tank), which is primarily earned by playing and winning games with premium tanks. Premium tanks are tanks which can only be acquired by spending gold, and there's a wide variety of tank types and tiers that they occupy. If you can afford it, buying a high-tier premium tank (which cost around $30-$50 USD in gold) can earn you a lot of Free Experience and especially credits (premium tanks earn considerably more credits than regular tanks) to help you power through the early tiers. Keep in mind that it costs gold (1 gold per 25 XP) to convert XP from your premium tanks into Free Experience that any tank can use, though.