User blog comment:JBox2D/Diana Rework/@comment-7709681-20141015160410/@comment-24090662-20141015222752

Identity-wise, I think this is pretty clear: she basically can't be an assassin since her damage numbers don't support a full burst combo. She's got 1060 (+ 250% AP) on her full burst combo which involves landing an intentionally unreliable ultimate, as well as autoattacking three times. She would be able to pull of a 100-0 combo if she's reasonably fed with a straight damage build targeting a squishy with no defensive items, but pretty much any bruiser built for straight damage can do the same thing. If you don't believe me, build straight AD Vi and watch carries explode by pressing RE. Also, the assassin side still has no actual disengage, and basically cannot kill anything without the ultimate, period.

The point is that her immediate burst is lower because she can't do her QRR combo to kill someone instantly, and also because her Q is significantly less reliable to hit (in exchange for her being less reliant on it to be useful). Her damage comes from the passive procs, which means sticking to a target and autoattacking them repeatedly, which generally means bruiser.