Another part of easy fun is exploration and variety. Some of the gameplay variety in Mario Galaxy includes:
-Flying with the bee suit
-Shooting fireballs with the fire suit
-Creating frozen platforms and ice skating with the ice suit
-Becoming a ghost who can turn invisible and float with the ghost suit
-Jumping very high with the spring suit
-Riding a manta ray on the water in a race
-Riding a turtle shell underwater in many situations, including races
-Balancing on a ball as you navigate through a level
-Flying with the red star suit
-Numerous tricks of gravity that vary across several levels
Just the moment-to-moment interactions involved with these things are fun, without even considering how they are used in the context of hard-fun-goals.
...
Camera
I used to think that moving the camera around while you are in the middle of platforming was part of the game in Mario 64. I was good at this, and I considered it one of the skills the game was all about. Mario Galaxy removes this "skill" almost entirely because it has an amazingly good camera system. Almost all the time, the camera is pretty much where you want it to be. This is a similar concept to the wall jump mentioned above, in that the game is much better off creating difficulty in other places than wall jump execution or camera fiddling.
Mario Galaxy's camera is actually an amazing accomplishment. I saw a GDC lecture one year about camera systems in games from the guy who did the camera for Metroid Prime. That game also has excellent camera handling (and the best mini-map ever). You might say, "But it's a first-person shooter! There is nothing to the camera."
What you don't realize is that Metroid Prime has over 20 camera modes. When you're in an open area, it's a regular first-person camera. When Samus rolls into a ball, it's third person. Some ball-rolling areas have a side-view camera and basically turn the game into 2D gameplay. Going through a tunnel has a special camera, and some boss fights have another camera.
A Mario-style third person platform game has even more demanding camera needs than Metroid Prime. In 1996, I would have not even been able to imagine a camera for a 3D Mario game that was basically in the right place almost all the time.
When you consider that Mario Galaxy presents far more challenges to camera design than any other 3D platform game ever, it's that much more impressive that it succeeds. No matter which way gravity is going or which kind of crazy thing you're jumping around on, the camera seems to know where it should be. This is undoubtedly the result of endless hours of hand-tweaking of camera paths and some very smart logic to boot.
