Yoshi's Story would have been an amazing game if it just had BETTER LEVEL DESIGN. The concept behind it was fun, and the graphics were amazingly good, but I felt that the levels themselves left a lot to be desired. There were really only a few good stages in Yoshi's Story (I liked 2-4, 3-4, 4-1, and one of the world 6 levels, but I forgot which). I know that you could go for melons if you want a challenge, but a lot of the "extra challenges" to get the 30 melon clears just weren't fun; that is, opposed to the extra challenges to score 100 points on a level of YI.
I think the problem with YS was that too many levels were unimaginative, and when that's combined with the "easy" way to pass stages, the game just lost all its replay value. That is, compared to a game like YI. Which was often the game it was compared to in reviews and the like.