Really looking forward to this, and all the while hoping the valve people are smart enough not to allow this to go f2p.
Basically a free to play moab ends up really being a free to phail (see what I did there?).
Dumb user who's never played before logs in and makes an account. Dumb user goes straight to a match - after all he doesn't need the tutorial or to try the game in an unranked mode. Dumb user then gets completely owned for the first 5 minutes after which he disconnects from said game. Dumb user repeats steps 1-3 several times until he can't rejoin ranked games because he has a higher than allowed LEAVER % on his account. Dumb user makes a new email at hotmail, and then a new account. etc etc..
This is what hon just did, many many of the original dota fans will know the pain this causes and realise where they went wrong.
Warcraft 3 also allowed this for playing strangely enough warcraft 3 - it was possibly THE main reason a commercial MOAB game needed to be made instead of it being a wc3 mod.
Accountability in this particular really nasty community is paramount.