I think the worst part about TNG is that they never revisited some of the former planets TOS went to, precisely to see what happened to them and perhaps show a different pov on the necessity of the actions taken by TOS. Piece of the Action, etc would have been most interesting.
They also dropped the Preservers/parallel development angle entirely, which could have been good. And instead of aliens using the Paramount period wardrobe (Romans, Nazis, etc) they went to the forehead widget. Ah well.