The greatest hurdle for Valve will be how to attract new users without hampering the gameplay. DOTA at it's core is an unforgiving game. New players learning fundamentals such as checking lanes and avoiding incoming ganks will get eaten up by veteran players. Deaths in DOTA are also huge morale killers and the heavy reliance on teamwork makes it hard for gamers to welcome new players.
If Valve can successfully tackle attracting new players without killing the core game, than they'll have another community-blockbuster.