Difference of opinion I guess. Mario 64's level's don't feel barren to me because I feel you aren't meant to wander them - most Stars in a level actually has a fairly linear path of obstacles to go through, and you can use Mario's various acrobatics (wall kicks, triple jump) to optimize and take shortcuts along that path. I don't think the sparseness of enemies or easy health regen (?) really matters as the essence of a platformer is in getting from place to place.
Then there's the fact its successors are better platformers. Sunshine warp levels are better than anything in 64 in my opinion. 64 is one of the most overrated games I've ever played and I LOVE me some Mario. I just don't love it.