Free to play, but with highly balanced set of heroes and items, the game is simply the best MOBA ever created.
Dota2 is based upon Defense of the Ancients from Warcraft III, but adds to gameplay and shortens the learning curve in multiple ways. Visuals are certainly much better, Dota2 TV and the Internationals add to the fun. the shop is more streamlined (takes some time to get used to it, but surely is better than DotA 1) and the new UI is a lot more user friendly. Slight gameplay changes are present as compared to DotA1 but those are due to limitation of Warcraft's engine, the spells now do exactly as they are supposed to do inside Dota2.
The game is free to play, but cosmetics items can be bought which have no effect on gameplay. So the game isn't pay-to-win style. The same cosmetic items have a chance of dropping for free in each game and a sure drop with player level ups (different from in-battle hero levels)
The fun events at Halloween and Christmas with drops in each game and unique cosmetics are also awesome.
There are a few bugs here and there but this is to be expected as the game is just released.
Over all, an awesome game. A must switch for DotA1 players, a must try for everyone else.