all of the above, its an rpg but leveling up only gets you tougher quests, instead of stats and abilites you go on quests and gather materials (sometimes just from gathering things from certain plants or rocks, sometimes from carving them out of the monsters you kill) that you use to make better weapons and armors, this way you cant become overpowered, as long as youre trying to find the right materials your armor should stay at just about the same "level" as the monsters you are fighting. there is also an online mode where you and a maximum of 3 other hunters can hunt a monster together, these online monsters are much tougher then single player ones but 3 companions keep it from getting to hard. online has all the same monsters, areas, and materials as single player (with probably a couple exceptions).
instead of having classes MH has weapon types, each with its own strengths and weaknesses, but you arnt forced to only be able to use one weapon the whole game, you can switch between weapon types whenever you want. MH3U will have 10 different weapon types for you to choose from. there are multiple versions of weapons in one type, one sword and shield might have strong fire element while another might have a poison status attack, there are i think usually around 20 individual weapons for each type/tier
monsters are large and don't necessarily go down easy. epic fighting where you must play smart. Once you get the hang of avoiding monster attacks and punishing said monster for missing you it becomes manageable.
The game doesn't hand much to you and you are self sufficient in the game world making and crafting everything you use. expect nothing but big boss battles against monsters with seemingly endless health. Combat is tight and the satisfaction of victory is oh so sweet. It takes a while to get to grips with things so just hang in there and stick to it. You'll see your skills improving in no time, but expect to get pounded at first.
Hop online and farm monsters for rare drops and expect to be entertained. --- Today is the Tomorrow you were promised Yesterday