Yoshi's Story's biggest problem was that it was following up YOSHI'S FUCKING ISLAND. So a super easy game where the challenge is to get a high score (which people stopped caring about back in 1984) was totally not what people wanted. I mark it as a big marketing brainfart on Nintendo's part. It's like they were unaware how much prestige the Yoshi brand carried at the time.
I have to concur with Ian 100%. I loved Yoshi's Island so much, and my anticipation for Yoshi's Story was off the charts (only Super Mario RPG 2, later to be called Paper Mario, was higher). For me, Yoshi's Story was a huge disappointment. The largest complaint being that it was so short, and in an era with few N64 games to tide one over that was a death sentence.
The game gets harder for the all-melons, and yes I got all 30 at one point. However, I did so more because I felt I had to get my money's worth out of the game rather than it being compelling enough to make me want to do so on its own.