This happens to every MMO, and ARPG now, I suppose. But I'm not sure I agree with the "great expansion" part. Act V was pretty short, and didn't feel interesting. Everything was kind of tacked on, not really having anything to do with the events of Diablo 3. Loot 2.0 and Adventure Mode was the bigger change, and that part shouldn't have necessitated an expansion to kick-start.
That said, I think Loot 2.0 is a major contributor to the fatigue.
The improved legendaries and their drop rates are helpful initially by giving you more useful drops early, but the end-game part of finding specific items with specific affixes is harder now without the ability to trade. So the early-to-mid difficulties/character gearing are much easier, but the later difficulties/gearing are much harder to accomplish.
Smart loot also makes nearly any item roll with a character's primary stat, except rarely, which gives less incentive for leveling alts that might be driven by a stash of gear for the wrong class, or to be used as trade fodder.
