I really don't know why people ever say this. Its like "omg, D3 will keep everybody hooked FOREVER!".
Uh, no, it won't. I'll play it from beginning to end, with friends, with multiple classes. Then you're pretty much done. I'll be moving on. And come back to D3 when expansions hit.
I still have no idea what the hell can keep people playing a game like this so long and feel like nothing else is worth experiencing. Its kind of sad, actually. This isn't even an MMO where you're never really done. This is a game where after a few months, you are actually, completely done. Its like watching a show over and over 10 times and you still wouldn't watch something else. Move on, for god's sakes.
When I'm fully done with D3, I'd be very happy for T2 to hit the shelves so I can see something new and different.