Well, lost levels used the flag as well. Mario games despite not using flags is actually something that people remember very fondly about the series.
I think, NSMBU is good, not great, but good and a fun game to play. The level design in it tends to be better than all the other games you mentioned except Mario World, but even then generally better, but is does seem like more of the same in a way.
I disagree completely about Mario 3D World though. It feels and plays very distinctive in comparison, but it it's own, not too much like 64/Galaxy or Sunshine, also not much like any of the 2D's ones but some similarities to all of them.
Gotta figure out how to set these up lol.











