A lot of people felt like it was missing a lot of the charm of the older 2d games; some of the worlds were typically ugly, the music was nowhere near the quality of World or Bros. 3, and while the 2.5d style I thought was pretty cool, it probably would've been regarded highly if it stuck to beautiful 2d sprites and backgrounds.
I enjoyed it, but honestly when I play it I always find myself just wanting to play World.