The major change in DotA 2 will be that it is going to be run on Valve's Source Engine which will completely remove all the Warcraft 3 limitations. All the heroes, items, terrain, buildings will be ported same as in current version of DotA. There will also be some things which are new in DotA 2.
In-game Voice Chat:
DotA 2 will have integrated voice chat feature which will allow gamers to communicate in-game without the use of a 3rd party software.
Leaver AI Replacement:
Disconnected players will be replaced by AI bots which will take control over the hero. It will be made available in unranked matches.
Heroes Voice:
New voice-overs. You will 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.
Rewards & Matchmaking System:
Steamworks will be integrated into DotA 2, what Steamworks does is it serves as a matchmaking & community rewards system which will allow users to create in-game rewards for participating in community forums.
Coaching & Training System:
Once again, Valve is coming with a great innovation of coaching system which will be used to train a beginner to learn DotA 2 to sustain a respectable position in the community. It will allow veteran players to login as a coach. Coach can see beginner's screen and can get private voice chat during the game.
It is indeed a revolutionary & surprise step taken by Valve by entering in the world of MMORTS games with the help of IceFrog. DotA fans will surely feel relieved after hearing some of the features of the upcoming DotA 2.
