This game is funtastic, great for co-op, reasonably good storyline, tho short, but overall reasonably fun and not too serious... I thought it was a nice casual game, the post ending gameplay is built around a more grinding based progression system, which isn't going to tickle everyones fancy... But if you treat it casually, and have friends to play it with I can see many hours ahead worth of gameplay and fun. Once you get to level 70 you get Paragons, and they not only help your character get stronger but are available to all new/opther characters you might have on your account, thus helping you gain experience faster for lower leveled players.
Overall I really enjoyed Diablo 3, and would easily recommend it, while the end game content is certainly a grind, if treated casually, played with friends and spend your time with moderation... Then you'll find the satisfying results of stronger gear, more paragons, and a progression which allows you to keep advancing, unlike other games that once you're done, you've unlocked everything and there's no replayability because there's nothing else to work towards.