Dota 2 is finally out for all to consume. Valve is releasing the game in waves in order to maximize game performance for everyone so people that are already in won't experience any disruption. You may have to wait just a little bit to download the game, be patient!
The free-to-play MOBA had been in beta for two years, and in that time has become "the largest active community in Steam's history." Dota 2 averages three million users a month, with a peak concurrent base of 300,000.