You make the best point about Diablo. Moo, you played the D3 beta, and if you felt it was unexciting I doubt the end-game content will change your mind. The endless killing for loot is what drives Diablo, not the final boss.
GW2, on the other hand, is entirely different from GW. I have plenty of blind faith in ANet but I don't know how it'll end up. There will be more Guild Wars 2 Beta Weekends in the coming months. If you like what you see and hear about this coming Beta Weekend you can always pre-purchase afterward to get into the next ones, if you want.
Some people were talking about pre-purchasing it from like a gamestop or something and then canceling their order, and theoretically you should still have guaranteed access to the other beta weekends (because physical pre-purchases require a different code for the beta and the full game). Not sure how it'll work out, but you could always look into that later on.