World of Tanks is an Excellent Example of what Free to Play Games Should Strive to Be:
The game has a massive amount of variety of tanks to choose from, all of which have both their strengths and weaknesses depending upon which nation you choose to go with. Most tanks are also very well balanced and even the premium tanks you can purchase don't have any significant advantage over the standard tanks, spare increased credit earning potential and lower repair costs after battle. Wargaming really goes out of their way to make sure most of the tanks are all usable in their own way, and have uses in combat so that no tank really feels like it is completely useless. There are a few tanks that are not very fun to use (such as the french Tier four light tank that is very slow and not very well armed or armored). But for the most part, every tech tree is easy enough to get into and continue forward without too much of a qualm.
With that said, progression really starts to slow down once you start getting into the higher level tanks. You are granted the tier one tanks right off the bat from all nations for free, and in a days worth of playing it is always possible to get up to tier four without much effort on your part. It also helps that you can exit a battle after you are destroyed, and jump right into another one with a different tank without the worry of forfeiting your earnings from the other match. However, once you get into the tier 5+ tanks, the amount of time you must invest into getting to the next level of tank (and even researching the parts for your new tank) can start to take an agonizing amount of time. For instance, the highest level of gun on a tier six T-34-85 from the Soviet medium line can cost as much EXP as it took to get into the T-34 tank right below it. Plus most tanks aren't all that fun once you first buy them, as the stock equipment is typically sub-par at best, and a fully upgraded tank behaves much differently and more effectively than a stock tank fresh off the line. This isn't so bad in the lower tiers when parts cost significantly less EXP, but once you are finally in that shiny tier seven T-43, you almost start dreading using the tank at first because you get placed in matches mostly in which the stock equipment isn't competitive against most players, and upgrades can take days if not weeks of playing at a time for only small difference each upgrade.
However, this slow progression does make for some huge satisfaction once you do finally start getting up into the high level tanks with all the shiny high level equipment. With that said, there isn't a huge advantage in having the next level tank with how the match making system works: Most standard tanks will be placed in matches in which all the other tanks are within two tiers of each other with few exceptions. So if you are piloting a tier three T-26 light tank, or a tier ten Maus with massive amounts of armor, you are always at least remotely competitive with the other tanks, and both endangered and a danger to all the tanks around you in some right.
Visually the game looks quite good: All the tank designs are very accurate as to what their blue prints or specifications were in real life. You can tell that the people at Wargaming do their research on their tanks, because they both look how they should and behave in a way that would be similar to real life. Of course tank combat isn't anywhere near as complex or dynamic as an actual battlefield, but it does offer a greater level of realism than any other previous tank combat game, and real life tactics definitely have an influence on how the game plays out. The textures are all crisp, and all the explosion and particle effects and quite well created. The sound design is top notch, and each gun has a different sound depending upon how powerful that gun is. Everything from the little PAT PAT of the smaller 20-50mm guns to the thunderous BOOM of the 120mm: all the sounds are realistic and have the proper impact to the gun that is firing. As far as free to play games are concerned, this game is absolutely extraordinary with its visual and audio design, and could likely even be competitive in the pay-to-play genre. I certainly would pay to play this game, and often do pay for the premium content (with over 100 dollars invest in the game to date).
With that said, the gold system does remedy the very slow leveling times in many cases. Gold serves a number of purposes that, while not offering any advantage during actual combat (except for premium ammo that is seldom used due to the exorbitant price, or slight premium boosters for the crew/ tank,) will have a massive impact on how fast you want to progress:
You can use gold to buy a premium account, which entitles you to 1.5X the EXP and credit rate for each battle you partake in. Additionally, gold can be used to buy premium tanks that are very cheap to run/ maintain and have much higher EXP and credit rates for each battle, while still remaining balanced and competitive. The EXP gained by premium (or Elite status tanks, in which all the upgrades have been researched) can be converted into Free EXP with gold that can be used for research on ANY tank. The free EXP combined with the premium account can exponentially increase how fast you level. Indeed it would even be possible to skip using a tank you don't like all together, and just use Free EXP to go right through the skill tree to the next tank without even so much as buying the tank you're doing research on. Gold can really ease the frustration in the slow leveling and stock configurations of tanks. Indeed a good model for bringing in the money without directly impacting the gameplay in a major way: Very clever move on their part.
The learning curve of the game leaves much to be desired as far as tutorials go. The game essentially has no tutorial, just a few tips and visual descriptions that point you in the right direction. When you first get into this game, expect to die a lot and likely have no kills for you first ten matches or so. Learning to pilot a tank is quite a difficult task, and it will be at least a week or two into playing before you begin to catch the hang of required tactics and how to use different tanks. Also realize that no tank you use is invincible, and even a player in a lower tier tank than you can easily hand your ass to you if you do not know what you are doing.
Content and update wise, there are very few people who I have seen who are as on top of things as Wargaming has been. They are constantly tweaking and adding new tanks, nations and tech trees to the game in periodic updates that happen every couple of months. They also have made a lot of progress in slowly improving their game engine over time, as well as the physics of the game. They really go out of their way to push the game engine to its limits and add valuable new content that has dynamic game play value. The amount of content that the game has is just amazing: Currently there are six complete nations to play as, each having their own different tanks lines and advantages to them. So whether you prefer the quick and powerful, but lightly armored Russian Mediums, the ponderous and heavily armored German heavies, the extremely thin skinned but nimble french lights or the quick firing and high penetrating guns of the British: There is a line of tanks for you, that has noticeable differences from every other nation in both visual style and game play.
So in conclusion, World of Tanks is worth a shot to just about anyone, and is a highly addictive and well designed game. This is a prime example of what all free to play games should be like, and I believe that it is a fine refinement on the free to play game genre. If you have played it: Keep doing so, because it is a great game. If you have not played it: You're missin' out dude.