Three major problems with SMS, IMO:
1) Blue coins
2) Super Mario 64 moved more effectively and acrobatically; the FLUDD just seems to be a drawback
3) Unmemorable levels and challenges
Otherwise, pretty decent game. Some things were definitely better than others, but it wasn't crazy amounts of fun (at least at the very beginning and hunting down every blue coin for the last few stars; middle's pretty good)