The Diablo III demos at Blizzcon 2008, 2009, and 2010 were specially-prepared for the event. Characters were pre-made, all set to a level that matched the content in the demo, equipped with reasonable gear, and with all of their skill points already assigned. The point in a demo is for fans to jump right in and start playing, and the developers don't want fans playing a brand new character without any skills or equipment, or spending their whole demo time looking over the skills and trying to decide how to spend their points.
Game features are included or excluded from demos depending on what the development team wants to show off at that point. For instance, Runestones were in the 2008 demo, but they were only enabled for Wizard skills. Runestones were not included in the 2009 demo at all since they were under heavy remodeling, but they were a major feature in the 2010 demo, though only for the Wizard, Witch Doctor, and Barbarian.
The game areas, quests, NPCs, and other features have also been custom tailored in the Diablo III demos. This was not the case with Diablo II; the playable demos they showed off at E3 in 1998, 1999, and 2000 were basically entire acts, with all the characters, skills, NPCs, quests, etc. The D3 Team has been much more conservative in showingg off the game, and all three demos so far have been small, self-contained areas of the game, with very little of the story or NPCs in place. No demos to date have shown off a town area, and only the 2008 demo has had an NPC (Captain Rumford) to speak with about more than the more generic quest details.
Features:
Multiplayer gaming with Blizzard's Battle.net service (players will be able to drop in and out of co-operative play)
Enhanced quest system with random level and encounters generator to offer different replay experiences.
Diablo III will be the first game in the series to use 3D rendering, but will still have an isometric, overhead view.
Interactive with environments: lay traps, use destructible objects against your foes, and use environmental obstacles to your advantage - all powered by the Havoc physics system.
New available classes and re-imagined old ones: Witch Doctor, Barbarian (more will be available as Blizzard releases more info)
