You only pay for cosmetic items and you get them as well from playing randomly. If you haven't checked out the game at all there's good competitive play to watch.
Here's what they've said about items: (sorry for the colour, it's cut/paste from the Steam news section)
"players who don't want to buy things from the Dota 2 Store will be able to earn them in a variety of ways, such as by simply playing the game, increasing their Battle Level, or by trading with other players."
“Dota 2 will not be a pay-to-win game. All the items in the store are cosmetic, and don’t affect gameplay.” “All of the heroes will be available free of charge. We believe restricting player access to heroes could be destructive to game design, so it’s something we plan to avoid.”
Also, they've said that the release of the item store is their final push to release so it may be that they're waiting just on a few more heroes and The International 2 tournament.