Rock Paper Shotgun
Dota 2 is the biggest game on Steam. No one else is close. This is why
Dota 2 isn't just a big game, it's a giant game. The number of people playing at any time dwarfs that of any other title, and it's not a close thing. This rather extensive article tries to break down the reasons for the game's popularity, and I felt like I learned a great deal while reading it.
We could be looking into a window at what competitive gaming will look like in the modern industry. Free-to-play, massive audience, and a game that acts more like a platform than a single release. Valve is doing a number of interesting things with Dota 2, and it looks like most of them are paying off.
I don't really understand the appeal of this sort of game personally, but it's always important to try to figure out why people are so attracted to this sort of game. This is a great place to start.