From the (very little) that I understand, it's a more streamlined experience than DOTA, but it's heavily based on DOTA, while DOTA 2 is a straight up ripoff of everything DOTA, including crazy Warcraft 3 engine limitations that became the norm.
While I understand that making a game like this more accessible is somewhat of a necessity (not really, I guess, right Valve?), I don't see how that can create a, as you said, 10x difference in userbase.
LoL is out in China while Dota 2 isn't, right?
Edit: Now we need to figure out this 5 million LoL userbase vs 40 million Steam users (hey, the game is free, every Steam user has the game, right?
). The more important number is the concurrent users. If it's 500 vs 1.5mil, that's more understandable.