The way I see it (and the way I see it is a very narrow and perhaps uninformed view, mind you), SEGA doesn't really know what the hell to do with Sonic franchinse anymore because they're constantly trying to appease all the fractured groups of its dedicated fanbase (the lingering retrofags, the youngin's who must continuously be courted for fresh blood to keep the franchise alive, ect) with seperate games. I can only suppose this is how stuff like Shadow the Hedgehog, Sonic 4, and the supposedly excessively kid-friendly Sonic Colours come about.
The thing is, some of the best platformers don't give an apparent damn about intended demographics: Super Mario Galaxy has both literal flying pastries over neon-blue skies and friggin' huge space fortresses with blazing guns aplenty. Yoshi's Island, Rayman, and the Megaman games seem fairly kiddish at first but have wicked difficulty curves that older gamers can appreciate.
But the thing about all the aforementioned examples is that they all possess very accessible gameplay, with limited regard to gamers' maturity or current level of skill. In doing so, such games increase their audience, their sales, and the profits derived from them. I'm no analyst in electronic media marketing, but I fail to see how catering to select audiences for specific games, especially for a franchise as well known as Sonic the Hedgehog, is as profitable or efficient as making each game with as much universal appeal as possible.
I think SEGA has an inkling of this mentality. The Advance and Rush series seem to be games that many fans, regardless of age, seem to be at least familiar with. And I'd say Sonic Unleashed fits in fairly well into the above examples of games with colorful aesthetics and challenging difficulty. So why not continue in that vein?
Granted, I've already over-analyzed Sonic Colours based on practically no evidence (the vague "the promotional trailer looks really colorful and kiddy!" description I keep getting from those who have seen it, and the uncomfirmed name "Sweet Mountain", to be precise). Perhaps the kiddy nature of the game will largely restricted to design choice and the gameplay will be able to be appreciated by all. As I've said before, the Mario franchise in particular pulls off that trick with astounding success all the time.
