That was well worth reading all the way through. You make some excellent points.
Sonic Adventure's relations to actual real-world locations intrigued me significantly, and even caused me to do some research on Mayan civilization, lightly touching on everything from customs to why they aren't around anymore. Even moreso, I was surprised they tied in actual Mayan mythology and symbolism throughout the game in addition to physical locations. In this respect, I'd say Sonic's connection to the real world was at its finest in this game. It still had a cinematic feel, though, this much was obvious. After that, it seems attempts to tie in aspects you mentioned have been nothing but downhill. Sonic Adventure 2 was pretty good, Sonic Heroes was tolerable, it was painful for me to play through Shadow the Hedgehog, I couldn't bear to finish Sonic and the Secret Rings, and although I can't rightfully have an opinion on Sonic 06, I guess it could have fared better. Everything has become convoluted, cinematic efforts over game play are increasingly obvious, and Sega's attempts to play into the latest trends have made things worse. I still enjoy most that were considered "spin-off" games, however, including Sonic Advances 1-3, Sonic Rush, and Sonic Battle. This might have something to do with the differing focuses in game play, music, and art styles compared to the more mainstream games.
Mario has kept his strength throughout the years, and whenever a major Mario game comes out, I never know what to expect: everything always seems creative and unique. Things happen that just don't make sense, and that's why people like it. Even Mario's RPG's seem to try to break away from the traditional formula, while still tying in core game play aspects. They always experiment with new graphic styles, and the worlds they create are always colorful and eye-catching, with subtle jokes and details that some people might not even notice.
I believe most of the strength of a game, assuming it has decent mechanics, lies in its art-style, musical score, and sound quality (including the presence or absence of voice acting). Real-world environments are beginning to feel bland and unexciting since they're used so often; this has really hurt Sonic's presentation as of late. Also, music in Sonic's more recent games (after Sonic Adventure 2) hasn't been necessarily bad, but it just feels like background music; it doesn't make me feel immersed in the game. Games with a unique style and musical score such as "The World Ends With You", "Jet Grind Radio", "Okami", and "Space Channel 5" have been received very positively by the public. They all tried new game play aspects, but they still made sure the presentation was crisp and fresh, which ultimately held the games together.
Regarding upcoming Sonic games, I almost feel that Sega is "ripping off" Mario with this new RPG for the DS, only trying an RPG with Sonic since it worked for Mario. Their approach of using recycled Sonic X artwork is questionable (tying back in to art being a key aspect in the success of a game), and I hope the multiple endings don't end up like what happened in Shadow the Hedgehog. Bioware has an impressive record, however, so hopefully they can still pull it off. I still don't know what to think of Sonic & the Black Knight, and would prefer not to jump to any conclusions this early on.