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."
The visual style is remarkable for retaining the somewhat cartoony feel that the Warcraft III version of DotA-Allstars is built around, while going in a few different directions. "I think there are functional aspects to the art that are pretty significant to the players," Johnson muses. The environment, particularly in the forests that fill in the map between the three lanes that the NPC armies follow, uses a desaturated color scheme to give the colorful heroes and abilities some visual pop. The sizable art team is putting a lot of work into making the shapes and animations of each hero distinct to the point that players will be able to instantly identify any hero they see and quickly gauge the threat level of any situation.
The game will also feature a ton of custom voice work. You'll get amusing lines from heroes as they deny the enemy team last hits on creeps, and champions who have backstory connections will trade quips when nearby.
The bulk of innovation in Dota 2, however, is ancillary to the gameplay itself. Valve is upgrading Steamworks (the company's backend technologies for matchmaking and other gameplay and community-related things) to allow them to create in-game rewards for participating in the Dota 2 community. The idea is to have everything a player does in or out of game tie back into their online identity. Like the improvements to Source, the Steamworks upgrades will be available to third-party developers who choose to use Valve's tools when Dota 2 launches in 2011.