I think I have been playing this game far too much. I'm into Special World 8 and I've only had it since Christmas Day.
It's a testament to the addictive power of Super Mario 3D Land. It might even boil down to minor presentational details, like how on the world map, the levels are previewed in those little dioramas, or the fact that worlds aren't separated by any kind of theme. I would partly attribute the 'just one more' syndrome I've had with this game to stuff like this.
Of course, the DNA of the Mario Galaxies is ever-present, and absolutely for the best, in my view. Several of the most memorable ideas from those titles - the flipping panels, the blocks changing to the beat, the morphing ground in the Ghost Houses - have returned with a twist, yet the new concepts - the directional platform-on-rails, the face snakes - also feel like they would have been right at home in Galaxy 1 or 2. It just goes to show that less adherence to world themes does allow for a level of variety that is rarely seen elsewhere.
Also, it's crazy that there are as many special levels as there are normal. One additional world I have come to expect, but eight? And just as much thought has gone into these as the regular levels. Even the Comet-style challenges like the time trials or the Shadow Mario stages work well, because they take things from the main game that you could play at a leisurely pace and then make you run. Fast.
I am slightly sad to have burned through 3D Land so quickly, but that's my own fault. Although I might wish it wouldn't end, the game is not want for quality content. Now if you'll excuse me, I've got some missing Star Medals to account for.