More points that I forgot:
-The world map is stale. As in, nothing happens on it. In SMB3, there would be hammer bros. that would block your way, card games that would appear, the super-secret floating money ship, the levels in world 8 where a hand would grab you to force you to play it. World has an interconnected map, and that's great, but it doesn't compensate.
-The potential of Yoshis with different abilities is squandered. They bother putting in Yoshis of different colors, but only the blue one is really that useful. Both the yellow and red yoshis serve nothing of value. Seriously, who here replayed those Star World levels to get a yellow or red yoshi? The devs could have given each different-colored yoshi some meaningful ability, and design some levels around these abilities, but no. Let's go further: how about levels in which different paths open up depending on the color/ability of your Yoshi? Why couldn't we have that?
-The game is piss-easy. I played through the game last night by not doing the palace switches until I was done with the rest of the game. It was the first time I played the game like that. Even then, the game was too easy. I barely got any challenge. SMB3, YI and NSMBU, on the other hand, offer plenty of challenge. Better-designed levels, for sure.