Honestly, there's a number of reasons I think 3D Land is trash, and I'm going to stand by them. The control scheme is atrocious, the levels further promote this ridiculous idea of 3D Linear Platforming, which is just broken by nature. The difficulty curve is nonexistent and the choice of adding Shadow Mario to the endgame of the Secret Levels was a bland and boring choice. The visuals are even less charming than New Super Mario Bros., the Tanooki suit is overused and cheaply so, there's a run button, the physics feel stiff, and the music sucks.
I get that it's all my opinion, but I honestly don't see how people could enjoy such a short, uninspired, and obviously rushed game. It's like, everything wrong with modern Mario in one game.
Interesting reasons. What is it about the controls that you don't like? I know you mention disliking the choice of implementing a "run" button, but to me it seems like a smarter design choice that would better fit the more limited movement of the circle pad. Other than that, I don't remember it controlling much different than other 3D Mario platformers.
I'm not sure that I understand how 3D linear platforming is broken by nature. I'd say it works quite well in the Galaxy games. Both games require you to make it from point A to point B while avoiding or overcoming obstacles. Comparatively, I agree that it doesn't reach the level of imagination found in some other 3D Mario games, and sure, the music could have been better, but I wouldn't call it uninspired, as the levels still exhibit a lot of creativity. The Shadow Mario mechanic was tacked on and one of the less enjoyable aspects of the game, but as far as it being short, it took me about 16 hours to 100% my save file.
I found Super Mario 3D Land to be enjoyable to the very end, and while not extremely hard, the challenge did ramp up as I progressed through the game. Everyone's experience is different, though. You may not think it's great and its presentation may not knock your socks off, but I don't honestly see how people can deny that its a good game. To refer to Super Mario 3D Land as "trash" gives me the impression that that person only plays "AAA" titles and that Super Mario 3D Land would therefore merely be "AA".