Okay, let me objectify it.
Visually: Sonic 06's aesthetic involves edgy cartoony anthropomorphic animals with practically photo-realistic humans. Sonic's hand is the size of Elise's face. Any of the NPCs' character designs (and most of the enemy designs) are devoid of any inspiration, and some of their animations are downright silly (watch the NPCs flap their mouths and wave their hands wildly while they talk). Nothing about the environment or world looks like anything that would normally belong in a Sonic game, it looks way more like Final Fantasy than anything else. The closest argument you could make is maybe SA1/2, but even they did wacky stuff with their levels and enemy designs. The graphics aren't that impressive for 360 either, way better shit has been done on the system.
Gameplay-wise: Pretty much every character is clunky to control. Attacks are rigid and lack fluidity, movement is unwieldy, the physics are generally haphazard and nonlifelike. At some points it's
just ridiculous.
Story: It's cut up weirdly with the gameplay, Iblis kills Sonic just for Sonic to literally be revived like five minutes later, the voice acting is lazy and the animations are still clunky.
This is all disregarding the long loading times and the numerous glitches that might make the game borderline unplayable. So that's what I mean when I say Sonic '06 is bad. Some people like it, and that's fine. Doesn't necessarily mean it's well-designed.
But as I said before, "bad" is subjective, so I respect you don't feel the same way about the game as I do.
Let me tell you something: I like Sonic R way more than I should. Sonic R is objectively shitty. Its character roster is grossly imbalanced, every character controls like a tank, the water looks weird and the game uses invisible walls too much, there's only five race tracks, and its soundtrack is... unorthodox. Why do I like it? I suppose partly nostalgia, partly the charm of seeing all the cutesy characters in a colorful 3D environment, and partly because once you get good at the game you can just pay attention to all its little quirks. But make no mistake: Sonic R is a bad game.
If I were to improve Sonic R, I would polish the controls, make turning way more responsive, rebalance the characters, add tracks, improve the visual presentation, improve the existing tracks and so on. What I like most about it is the concept, the characters, and some of the unlockable objectives. But I'd probably be better off redoing the game from the ground up.
To me Sonic '06 is pretty much in the same boat. I don't know what about the game is so redeemable that someone couldn't just make a better game entirely from scratch. Pretty much the soundtrack is good and that's about it.
