Although you bring up some good points chris and Machenstein, I think the main problem with Sonic Heroes was that the level designs were utterly atrocious. All of the levels are essentially elevated space tracks with the occasional on-rails sections like in Sonic Adventure 2, but made worse
with the fact that the entirety of the levels were surrounded by the killer pits of doom. Given the small amount of running space and the linearity of said levels, the platforming was completely dragged to the ground. Honesty, rolling down a pathway trying to avoid fatal pitfalls whilst simultaneously fighting with dodgy movement physics is not what I would consider fun gaming. The switching characters gimmick was pretty ill conceived as well; it didnÃ¢â‚¬â„¢t make the game more exciting to have to use specific characters to get past certain obstacles, or to collect power cores to destroy Eggman's robots more easily, all it did was slowed the game down.
I still have no idea why this game is considered by some as a 3D version of "classic Sonic". Graphically maybe, but the gameplay of Heroes was so completely against the design mythologies of the original games that in reality, there should be no comparison between the two whatsoever.
The audio qualities sucked too. The music is the worst Senoue has ever written and the characters just won't shut up.
I'll agree with the charracters being annoying to listen too, but I still think the music was quite good. Well, better than SA2 at least.