The playability has improved greatly since the original DotA. The UI for DotA 2 has been tweaked to be far more friendly to the novice player. Items used to be one of the most confusing elements of the game and has been fixed by giving each player a simple to follow suggested items guide.

Starting a game is also incredibly streamlined. Three or four clicks, a "time until in game" suggestion and then that's it. They've also introduced a karma system where you reward or punish other players for their actions, including language, suicidal playing style and so on.

I agree it'll be interesting to see how it competes with other games in the same genre but my money right now would be on DotA 2 simply as Valve really know how to make their games.