the story being bad is an inarguable point; it isn't actually About Anything and imparts no moral or message, and as pure-entertainment B-movie schlock it fails to be entertaining and is just awkward creepy furry fanfiction; it's all literal events (OH NO A BAD THING HAPPENED, : SAD : ) by someone who doesn't "get it" without any sort of worthwhile context. It doesn't work as a "real" story and it doesn't work as "just fun" because it's TRYING to be a "real" story. the level design is horrid, lacking any of the kind of frictive, joyful build-up of speed present in earlier sonic games that had the good mind to keep it scarce, too, understanding that scarcity makes things more valuable. the music is merely serviceable at its high points, eye-rollingly bad (for anyone finished with puberty) buttrock at its lowest points. the graphics lack any kind of interesting style; the environments are bland (compare to the surreal, wonderfully gaudy pop art look of Sonic 1 and CD, especially the latter), the character designs are overly, awkwardly lanky and "anime"; the robots lacking any kind of recognizable traits, having been reduced from colorful evil lookalikes of various cartoony flora and fauna into generiscifibots 2.0; the side characters looking like something a 13-year-old on deviantart could've done, literally; and the uncanny valley almost-realistic-but-too-almost-for-its-own-good humans very awkwardly clashing with the anime-y bright-and-saturated cartoony animal characters.
give back a sonic game with minimal story, gorgeously colorful graphics oozing style in an aesthetically-appealing presentation with catchy music that fits with the aesthetic of the visuals featuring genres other than buttrock or overblown-american-action-movie-orchestra (or some dreadful fusion of the two), level design that takes advantage of sonic's superpower of speed as, well, a superpower, giving it a context that is meaningful and rewarding (see: the many devices through the genesis games that require sonic's unique ability to Be Really Fast in order to work which allow him to traverse portions of a level) as well as not shoving it down our throat every second (and since speed will be something careful and measured, there will be room for naturally-placed slower, more calculated platforming segments, thus removing the need for having to play as other characters to get that) which in turn with having to actually Work For It will make the speed feel more special and worthwhile; also, more measured speed distribution can make for levels where you can actually explore and shit, which can be interesting, and robot designs that have as much personality as the rest of this dream game
(btw Sonic CD meets all this criteria and more, Sonic 2k6 meets none of it and therefore in order to be a good game Sonic 2k6 would have to have so much changed that it would essentially not be Sonic 2k6 anymore)
(this is a really long post so some of it might be a bit rough i'll read this over and make it less stupid in places it needs fixing at a later time)
edit: I almost want to expand on this even more because some of this isn't explained well enough I'm wondering if i should make my own topic
give back a sonic game with minimal story, gorgeously colorful graphics oozing style in an aesthetically-appealing presentation with catchy music that fits with the aesthetic of the visuals featuring genres other than buttrock or overblown-american-action-movie-orchestra (or some dreadful fusion of the two), level design that takes advantage of sonic's superpower of speed as, well, a superpower, giving it a context that is meaningful and rewarding (see: the many devices through the genesis games that require sonic's unique ability to Be Really Fast in order to work which allow him to traverse portions of a level) as well as not shoving it down our throat every second (and since speed will be something careful and measured, there will be room for naturally-placed slower, more calculated platforming segments, thus removing the need for having to play as other characters to get that) which in turn with having to actually Work For It will make the speed feel more special and worthwhile; also, more measured speed distribution can make for levels where you can actually explore and shit, which can be interesting, and robot designs that have as much personality as the rest of this dream game
(btw Sonic CD meets all this criteria and more, Sonic 2k6 meets none of it and therefore in order to be a good game Sonic 2k6 would have to have so much changed that it would essentially not be Sonic 2k6 anymore)
(this is a really long post so some of it might be a bit rough i'll read this over and make it less stupid in places it needs fixing at a later time)
edit: I almost want to expand on this even more because some of this isn't explained well enough I'm wondering if i should make my own topic