I bought Diablo 3 on midnight release and played it A LOT for the first few days. I hit 60 and was immediately bored after going through the storyline multiple times and being scarcely rewarded.
People could use the argument that Diablo 2 was exactly the same, but there were a few key parts that made it differ in terms of benefit.
I played Diablo 2 for at least 9 years and I still enjoy it. Heck, I uploaded some videos of me getting rank 1 on the U.S. West Ladder on May 2, 2011 lol. The problem with Diablo 3 is that it does not contain key parts of Diablo 2 in it that made the series fun in the first place.
One example is the itemization. I enjoyed seeing a unique ring drop off Andariel. It could be various options whether it was Nagelring, Manald Heal, Bul-Katho's, or the Stone of Jordan. These items could drop off any act in the higher difficulties and based on the level of item even in Nightmare. In Diablo 3, it's focused around rares and hardly legendaries let alone all of the good items have a severely lower drop rate compared to Diablo 2 because of the Auction Houses. The game is about loot, and the loot isn't very rewarding in the third game.
Another option is Player vs. Player. Anyone could duel in the Blood Moor starting at level 9. The fact there is no open area to do so and only arenas will be implemented let alone almost no focus or balance will be put in to favour this aspect of the game just destroys the series entirely. Diablo 2 only lived as long as it did because of the Player Killing.