I say his launching isn't great and you say his side tilt is "decent" (I think that's generous but OK) and that his up air is good. His up air is good, but again that boils down to how he's used with juggling. It's also exactly one move that you've named that's a plus in the launch department. He's unquestionably good at the low-air game but I think his ability is narrow. You say his specials do all these things, but his side B, for example, moves quickly and in a non-standard arc. Does that mean it's bad? No, but that does mean there's only so much one can do with it in a pinch, hence utility. Same with his neutral B: maybe it can do two things you'd like it to do, but I don't know if there's another projectile in the game that leaves the user so exposed. Does that mean it's bad? No, but it does mean there are very few opportunities to use it effectively. Diddy's moves are fairly narrow in their application, which reinforces his juggle game -- it's effective and you can break it out in most any situation. Diddy doesn't have much of a secondary strategy -- as far as I know no one really plays him except to juggle -- so when (I don't think it's if) the metagame breaks down the Diddy juggle, he's not going to be looked at as that same sort of strong character.
That's why I say I think the game will grow beyond Diddy, and that's why I compared complaints about Diddy to the first-week complaints about Little Mac (which as you said correctly were largely due to the way For Glory works, absolutely). Time will tell if Diddy needs to be weakened or if the character's usefulness is just too narrow to stand up over the long run. My bet is on the latter, but you're free to place yours elsewhere.
