People were just used to having multiple of the same class with different builds, because that's the way you had to do it in D2. D3 just allows you to change your spec at any time, eliminating the need to ever have more than one of a class. I don't know about the "linear" comment, but I guess that has to do with the fact that most (if not all, I don't remember) of the areas aren't randomly generated.
D3 is just a lot easier than D2 in general, and a lot of old-time fans don't appreciate having their hands held. The game was developed by a completely different dev team, and apparently they had "if games are easier, more people will like them" too far ingrained in their minds from the development of WoW to do anything else.