To say the truth, my critics were for NSMB2.
NSMBU looks fun and entertaining.
This is one of the games that actually make me wanna buy WiiU.
NSMBU, Rayman Legends, Assassin's Creed III and Bayonetta 2 are the games that most interest me now on WiiU.
But before buying WiiU, I'll wait a couple of months to avoid defective units.
Unless something SERIOUSLY catches my attention, I'll probably wait at least until early next year to grab a Wii U. Mostly because I just got my Wii this April, so I have tons of Wii games I still want to play through (I'm juggling Metroid Prime Trilogy, Super Mario Galaxy, and Skyward Sword right now). My girlfriend and I just recently finished playing NSMBWii together, so I might grab Rayman first, just for a change of pace. But I'll definitely be getting both.
I honestly hated playing co-op for NSMBW. It just felt very counterproductive. I'd go to jump on a platform and my friend would jump on me to send me to my death. Plus the game gets way too easy with two people. One person dies, the other just waits around and gets you back up. It also suffered from issues with distinguishing characters. Especially when it comes down to two Toads that are fucking identical outside of colors.
I felt that Co-op was much harder than single player, for the reasons you mention in your first 3 sentences. It was easier in a lot of ways, but those were offset by the new set of problems brought about by co-op. The one thing that truly bothered me about co-op was how the game would "freeze" whenever one of you got hit or died. So many times I would be timing a jump and hit the button RIGHT as the game froze, but the button press wouldn't "carry" after it froze, causing me to fall to my death. This pissed me off to no end, and I hope they got rid of it in NSMBU.
It was a love-hate relationship for me, honestly, but most of it I felt had to do with poor control. The wall jump was a near-useless move that shouldn't have been in the game, and caused so many deaths and mistakes (I'd try to jump up a high ledge, and Mario would grab it instead and my only option would be to leap off, often leading to my death). Also, the ground pound should've been relegated to the otherwise unused B button, or the minus button, as just pressing down in midair caused so many deaths (like trying to grab the fences in castles, and ground pounding into the lava instead).
I never played 4-player co-op (just 2 player), so I never had issues with distinguishing toads. But either way, since Mario and Luigi are just color swaps with different thickness and height, I can't imagine it being much more of a problem.