Personally, I myself am an ASSFAGGOTS(Aeon of Strike Styled fortress assault game going on two sides) Player. I.E Warcraft 3 dota / DotA2.
What I find really fun about this genre of gaming, is the fact that every game, while the map is the same, feels different. Some can be easy, some can be hard, some just laid-back and chilled.
But it's not the difficulty factor that brings me in, else I would just play games with artificial difficulty - I.E. Dark Souls.
It's the feeling you get playing the game. You are caught up in the game, giving all you can give, knowing that when you win those hard games, you get a sense of euphoria from it.
That's what I need in a game, something that brings you back, rewards you for winning.
I don't like the way a lot of games are going at the minute though, Pay for in-game items that give stats (I.E. Free to play MMO's). Then your euphoria just comes from your wallet.