In terms of gameplay, I should say that the guys over at Gaijin didn't quite do anything too out of the ordinary I guess. The available options or features are pretty much the run-of-the-mill types that you've come to expect from a game of this genre.
Progression within the game is just like that of any other lobby-based kind of game. You play or grind the game for some form of currency and then use it to upgrade yourself or buy access to better equipment/gear. Or in this case, planes...
Don't worry, your not country-locked, so you can play whichever you want, as and when you want it. You may even tech them all up if you so wish to. In fact, your strongly encouraged to do so as most of the single missions and dynamic campaigns would require you to unlock at least "certain" planes before allowing you to partake in them.
However, do be warned that these so-called upgrades are available and open to each and every player who flies that particular plane. So long as its the same plane, and that the players have all flown them for a sufficient amount of time (so as to acquire all upgrades), they will be flying them on a leveled playing field and everything's down to the pilot's individual aim and/or skills.
(More so if their flying different planes...which will then require them to know the quirks and fancies, advantages and disadvantages, of their own planes as well as their opponent's.)
Perhaps the most interesting part that caught my attention in War Thunder would be this function known as "Difficulty Mode" or "Game Type".
They can fire their weapons without a care for ammo conservation. Emptied guns automatically refills itself after some time. Jammed weapons can magically fix themselves whilst even flying in mid air. Diving down from high altitudes, at extreme speeds, doesn't rip off your wings. Taking non-critical wing damage to your plane doesn't affect your flying performance one tiny bit. You name it.
Players need to refill their ammunition, unjam their weapons and refuel at an airstrip or carrier. Wings can get ripped off if you aren't careful with your speeds, pilots can get blackouts if you do too tight a turn and damaged wings could cause all sorts of malfunctions and hindrance. (Like how my plane keeps rotating anticlockwise and I had to compensate for it in order to fly straight.)
As for simulator mode, if I'm not wrong, players get thrown into a full realism kind of gameplay. No markers will be shown on the HUD to keep track of your targets, there won't be any third person view to fly your plane in and everything that is found in realistic mode will be inherited as well.
Its "virtually" as close as you can get to flying a real plane. And according to what I've heard, players who flew in this mode highly recommended the use of a joystick.
But be warned. Compared to arcade mode, realistic and simulator modes are much more time-consuming to play. Furthermore, because of that, the population of people who actually play these modes is quite small and thus the average time taken to get into a match might be correspondingly longer as well.