First of all, Yoshi's Island is one of my all-time favorite games. Beautiful graphics, intricate gameplay, super-cool bosses and *GASP* an interesting storyline in a Mario game!?
Seriously, I see no "fluff." There was a decent challeng in finding all 100 points on each level, and some of those secret worlds were flat-out devious. Don't call something "fluff" just because it was release within the past 6 or 7 years.
Yoshi's STORY, on the other hand, was a complete pile of steaming, "fluffy," donkey crap.
Yoshi's Island, to me, is just as "classic" as Mario Bros. 3 and Mario World.