Observation: after taking my sweet time with Galaxy 2 I've come to the conclusion I like its progression best of every Mario game, /and/ Yoshi's Island. (And it takes a lot for me to place something above the Yoshi scoring system for its stages.)
The way this game is laid out reminds me of a combination of several other games' concepts:
1. Mario 3's discreet world map screens.
2. Mario World's layered and interlocking secrets.
3. Yoshi's Island's more linear map progression tempered by going back to stages to increase your completion / exploration.
After the huge response to New Super Mario Bros. Wii, many thought that the oldschool worldmap was a quick fix to make it look like NSMBWii at a glance for "the masses". Now though I don't think this is really the case - NSMBWii probably made them reconsider the map system, yes. But I really think they realized that the presentation of a Mario game and the pacing of how its content is unlocked to the player is key. It's not just about having a bunch of random levels floating tucked away around a hub.
What's nice is the way progression never falls into a predictable pattern. But without relying on one too many gimmicks to pad out the overworld presentation. The worst part about NSMBWii was the unnecessary "encounters" that echoed the SMB3 Hammer Bros. without really serving the same function. Those just padded out the map to make it seem as if more was happening (and more was interesting) than there actually was. Galaxy 2 by contrast is very lean - and it helps that the game streams like gangbusters, with basically zero visible loading times even when zooming into the starship microhub. Of course, it helps that there are more overall galaxies in Galaxy 2, populating the world maps with just plain more content.
Speaking of which, major props go to the starship itself, a "microhub" being an interesting and basically non-intrusive solution to giving the player a sense of home base and something to play around with should they wish. The fact that the game doesn't default back to the overview map after exiting a galaxy is a toss-up. It doesn't really take more than a jump forward to get back out to the main map, but it would have been perfect if they'd given you the option to default to one mode or the other upon STAR GET.
The only remaining presentational flaws in Galaxy 2 the initial infernal chattiness of uh, Space Grimace as he insists on getting in your face about every little thing. Nintendo's attempts to childproof, idiotproof, and Brain Age-crowd-proof their games doesn't do Galaxy 2 any favors. Ironic since the Galaxy games are some of Nintendo's most "hardcore" titles at their elite levels of play and in their full depth and breadth.
Despite this small blemish though, Galaxy 2 has become the most inviting Mario game for me to play and a further lesson from one of the very few developers to fully understand the importance of presentation and treating the user like a "king". The smallest detail matters, right down to the fact that Starship Mario doesn't just slide from point to point on the map, but turns and chugs leaving a plume of steam behind it.
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