Diablo guarantees you a legendary the first time you kill him (after loot 2.0)....you've gone 40+ hours and haven't reached Diablo yet?
That got patched in later, it was originally the Skeleton King that had a guaranteed Legendary drop. I know my husband didn't get a legendary on Diablo the first 2 times he killed her (spoilers sweetie ;P ) since loot 2.0.
He has had a bucket of legendaries drop overall though. And at least 1 set item. I got a plans for a L21 Set the other night, not sure if they would be worth making for an alt. Given how quickly you level now, they would be replaced before I could earn the materials back. Might make them just to have them in the transmog panel.