You don't have to go far to find people gushing about Super Mario Galaxy. Any nerd with a Wii has probably spent a lot of time indoors this weekend playing through it. I haven't seen a lot of people commenting on some of my favorite aspects of the game (probably because they're not as sexy as talking about the graphic design or the levels).
The Scale - There's fifteen stages with at least three layouts and then another thirty-five stages that are one shot puzzles. I got more than twenty hours of play out of exploring the stages and getting all 120 stars and I'd like to think that I'm a somewhat skilled player. It's the right scale for a game for me; long enough that I don't feel cheated but not packed with repeditive, boring filler to extend time (Assassin's Creed, I'm looking at you). With that many stages there's a wide variety of things that a player will be called upon to do and almost each one has a distinct feel. The one unfortunate exception is a stage near the end which uses most of the same geometry as a stage near the beginning, and not in an ironic revisiting way.
Most of the stages are more linear than those in Mario 64 but there is enough exploration to let players dig around for hidden bonuses. Since the bulk of the environments are mapped onto three-dimensional surfaces there is a deceptive amount of area even in places where it doesn't look like there's much.
The Interface - Think about this for a moment. You have a game with an environment where some objects attract the player to any surface as down and others where the gravity pull is in one constant direction. Some places where falling is fatal and others where the player is attracted back to the nearest surface. How do you do this and make it coherent to the player? The answer that the developers of Super Mario Galaxy came up with is some subtle graphic design that gives obvious clues. Do you see a swirling black hole in the background? Don't fall there. Is the edge a sharp right angle? Then you can't change gravity there but you can smoothly cross over any curved surface.
The game doesn't use a lot of the Wii's motion tracking but it uses it appropriately. The waggle attack and activation is becoming a standard feature for games that don't have a lot of use for complicated motion tracking but the real gem is the fact that the player has a mouse cursor which can be used to collect power ups and shoot enemies to stun them. A player with a lot of FPS experience would have no problem shooting enemies while walking Mario around with the nunchuck's analog stick.
The Co-op Mode - Co-op play in Super Mario Galaxy features completely different game play for the second player. Instead of the traditional platforming that the first player is doing the second player is essentially there as an assistant. They have their own cursor which can be used to collect power ups, fire bullets, stun enemies, and help the first player jump higher. The game is still the first player's show but someone else who is watching can just jump in and play along. It reminds me a lot of Sonic 2 where the second player could take control of Tails at any time to assist the first player but the first player was the only one who could move the viewpoint. No one is going to get Super Mario Galaxy for this mode but it's a nice bonus.
I wouldn't call Super Mario Galaxy the greatest game ever but it is a lot of fun. Anyone who has enjoyed a Mario game in the past is sure to love it.