Well in that sense, Sonic has been making huge steps towards consistent quality. Unleashed was half great, Colors was a critical success, Generations is looking to take the best of Unleashed and Colors and mixes it with the best elements of the past 20 years. Reading the many E3 articles, I seen countless mentions of the previous title (Colors) being great and Generations continuing that trend.
Like it or not, the "buttrock" you mention has been a major part of SEGA games since the Dreamcast era and as cheesy as it is, a lot of people enjoy it and it is jut as defining to the Sonic games as super saccharine overplayed tunes are to Mario. Again, I really think that aspect is a non-issue for most people and you just take great offense to it.
What makes a "bad story" is up to personal opinion, but I'd say stories that take far too long to tell in too many cutscenes constitutes as bad. Also, too many plotholes are no good. Sonic Adventure 1 and Sonic '06 are the most guilty of this. Sonic Adventure 2 has, in my opinion, the best told story in a Sonic game and does so in quite breezy cutscenes. It's not perfect, but it's fun. Heroes and Colors are quite simplistic stories, and while neither are gripping, they are suitable for a Sonic game in my opinion. They're no more complicated than the Genesis games or a Mario game for that matter.
Glitchy gameplay is something that Sonic will get over in time, as long as they release a string of main series titles that are playable. I don't think a 3D Sonic game will ever bee completely glitch free, due to the speeds at which Sonic moves. It's a very tricky thing to get right. Mario seems to stable because he moves so slow. Of course his games are going to be easier to perfect.
Half baked playstyles died out with Unleashed, in my opinion. Unless somebody takes great offense to the Wisp powers, however I had no issue with those and consider them hardly half baked and thought a majority of them were a lot of fun (drilling, rocket and laser especially).
As for Mario being "solid as a rock", I find Mario's solidness to be bland. Sure his games are fun, but they're a very safe fun. I also hate Mario's speed and gameplay mechanics. Of course, that's a personal preference. I know a lot of people like the titles and consider them to be a consistent quality. I just hope the new HD Wii will push Mario beyond the Galaxy/64 gameplay and into something more exciting.