Well I just got all 120 stars and I'd say it's the greatest Mario I ever played, putting nostalgia aside.
Swimming controls definitly annoyed when trying to get out of the water, but I never died or lost a challenge from it. I felt the difficultly went up nicely, from anyone can do this up to pull your hair out a few times but you will still get it eventually.
As for the 'co-op', what they came up with is great for two people to work together instead of being bored and waiting for the other person. What I see being discussed about is people did not want a co-op mode, they wanted a competitive mode, which is not in the scope of the game which is why I feel the arguments are out of place. (this goes for other places I've read complaints about it too)