I like that there will be certain amount of randomness to the play areas, but I am hoping that extended play (meaning extra rather than just Easter Egg "secret" levels) will be a part of the NIGHTMARE and HELL difficulty levels.
One of the greatest weaknesses of DI and DII, as well as the many "clone" type games is that the second and third difficulty levels are exactly the same as the Regular or Normal difficulty levels. Even with some randomness (As there was in Diablo I), part of what makes the game great is the sense that you can "find" or run into some place ( or some horror) that you have never seen before.
Due to the lengthy wait for this game, I hope that Blizzard plans to address this serious weakness in the series and add at least 1 or 2 "exclusive" areas for both the Nightmare and Hell difficulties, both as a reward for the dedicated players who "slog through" the higher difficulties, but also as a means of maintaining tension and "terror" as part of the atmosphere of the game. While I know that there will be class specific quests and things to find, what I am hoping to see is difficulty level specific content (Perhaps also random, but random specific to it's difficulty). Ideally, even an experienced player who has completed Normal, Nightmare, and Hell levels with one of the Character Classes, should be able to play a second class and upon getting half way through Hell with a second class, should have the feeling that behind any door, or portal, that they could stumble into a new and terrifiying area where creatures and bosses that have never been seen before could emerge to surprise and terrify the player.
If Blizzard were to add this to what already appears to be an excellent and groundbreaking game, then Diablo III could be as close to a "perfect storm" as would be possible for a PC game.