I have to respectfully counter-point your last sentence. There's too much to do. The map is a cluttered mess and I find it very annoying.
Combat has been changed, you can no longer grab an enemy to use as a bullet shield when you are being shot at. Also enemies are smarter so you don't want to engage more than 2-3 at a time, if even that many. Arno becomes the first character who cannot slaughter an entire army all at once. I'm very mixed on that. I like it but I don't.
Co-Op is here and for the most part I like how they've implemented it.
Customization options are through the roof. You can personalize your Arno a lot, and as you play through co-op you'll find no two Arno's are alike. Lots of customization for appearance, weapons, and all that great stuff. Probably their best tweak on this game. It's essentially all the multi player customization you could do for your characters but now in the single player, with even more options.
Miss the multi player, but as much as I loved it I suppose it was stale and time for it to go. Black Flag was the first one where I didn't play a character to lvl 50.
Also, to counter some of the high-profile media reviews, I don't think the story is that bad. I'm enjoying it.
That's my 2 cents. Not their best work, but not their worst either.