SMG really is a textbook lesson in how to please everybody via a single game. Just take a look at the various level types and who they appeal to...
Normal levels: Everybody. The difficulty ramps up slowly enough that even a beginner can beat the game with a minimum amount of stars, and I can't imagine even the most hardcore gamer not enjoying them.
The last 3-4 galaxies and the prankster comet levels: More skilled players who like an extra bit of challenge.
The secret stars: Exploration fanatics.
The purple coin levels: Hard-core platformer enthusiasts who actually like collectathons.
Thus, Miyamoto and Nintendo created a game with core content for everyone, and with enough extra content to please a wide range of different groups. It's a lesson that other game developers should learn from - including Nintendo themselves, if they want to keep toeing the line between casual and core.
"'Casual games' are something the 'Game Industry' invented to explain away the Wii success instead of actually listening or looking at what Nintendo did. There is no 'casual strategy' from Nintendo. 'Accessible strategy', yes, but ‘casual gamers’ is just the 'Game Industry''s polite way of saying what they feel: 'retarded gamers'."
-Sean Malstrom
