DotA-Allstars' roster of 100+ heroes is being brought over in its entirety. The single map games take place on is functionally identical to the one that you can download for free today in the Warcraft III mod. Items, skills, and upgrade paths are unchanged. Some hero skills work slightly better due to being freed from the now-ancient Warcraft III engine, but Dota 2 will be instantly familiar to any DotA player.
A few things will make significant differences to players making the transition. Dota 2 uses Valve's Source engine, so the game is much prettier. Source itself is getting a few upgrades, including improved global lighting and true cloth simulation. Dota 2's integrated voice chat is a huge step up from having to set up your own Ventrilo server, and the speed of voice communication is very nearly a requirement for a game as team-focused as DotA.
AI bots will take over for disconnected players, and will be available to play against in unranked training matches as well. However, don't get your hopes up for a full-fledged single-player game, though. Johnson says, "Our goal with the AI is just that their experience isn't destroyed just because one person couldn't finish the game."