Super Mario Bros 2 took a risk since it was entirely different than it's predecessor, but still managed to become a classic and well received game.
In my opinion, Yoshi's Island took that same risk, and again, Nintendo managed to cement a classic.
It's vastly different than the first Super Mario World title, but all in great ways (aside from baby Mario's crying!). The artwork is beautifully unique, going for a more cartoonist hand drawn style. The gameplay is addicting, adding several new elements from the previous Super Mario world.
In simpler terms: This is still a platformer through and through, but if you expect similar gameplay to to Super Mario World, you may be disappointed.
Can't recommend this game enough.