Mostly because at the time of the original release people were sick of the "New" series and art style. It was the 4th in 6 years and only 3-4 months after the 3DS release. People wanted Nintendo to mix things up more.

Except we're far away from that, now. Shouldn't we take a step back and judge a game based solely on its merits instead of comparing it to our expectations from more than half a decade ago? 

Super Mario World is one of my all-time favorite games (it's in my top ten of all time) and even I have to admit that NSMBU is, from a gameplay and polish perspective, almost as good. It will NEVER have the nostalgia or aura of mystery around it like SMW, and the soundtrack is sort of bland by comparison but from the standpoint of design and world building and gameplay? It's on par with the best. 

I agree.  The Feb.13 Direct was awesome and didn't even include Pokemon (which got it's own Direct).  I also think the Sept. 2018 Direct was exceptionally good.  

But really this is what I expect from Nintendo Directs and E3 going forward.  Third party releases are only going to increase on the Switch.  On top of that all of Nintendo's home and handheld teams are making games for Switch.  They don't even have the 3DS to drain away resources at this point.  It's just going to be a ton of Switch games on every "big" direct going forward.

The difference is that Super Mario World was made for one player. NSMBU was made for multiplayer and I really got the feeling that this is not for me who wants a single player game.

I think most recognize that it was a good game, the criticism at the time was that it was too similar to the DS, Wii & 3DS releases and the criticism now is that it's a 6 year old port instead of a new title.

Yeah Nintendo kicked it out of the ballpark with this month's announcements. I wouldn't even be shocked if we didn't get a full sized general direct before E3, because the announcements were so good.

The only thing I hope is that Pokemon Sword and Shield has the time it needs from now to release date to polish things up and maybe even add in some crazy features

Now THIS is a good assessment. NSMBU is definitely designed with multiplayer in mind, but you don't HAVE to play it that way. I've never played it multiplayer and I'm fine with that. I do agree that it may make the game less appealing to some people, but I still think people should be a bit more open-minded about it all. The game's a tonne of fun.

Having seen the price for DOA6's season pass one has to now appreciated how Smash is being handled holy shit.

Even if we set aside the NSMB fatigue of its original release and the fact it's an ancient port on Switch, I'd still say NSMBU is just a mediocre game. 

Level design is merely okay, graphics are about as generic and soulless as you can get, the music is atrocious, and not once in the entire thing does it have a truly inspired moment of brilliance. It's Nintendo at their most complacent and lazy. There's not even a spark of the passion and magic you see in games like Odyssey.

Agreed. I enjoyed the game a lot on the Wii U and I still enjoy it on the Switch. I can definitely understand the artistic direction criticism, as I was feeling a bit of the NSMB style fatigue myself. Even taken on its own I find the style to be a bit sterilized and plain (though better than previous NSMB games). However, I enjoyed the rest of the game too much to let that bring down my opinion of it.