I liked Sonic Adventure, I even enjoyed the Adventure Fields. They were flawed, sure, but it was nice to have an open area where you could run at full speed. Instead of making the levels narrow and racetrack like there should have been more levels that were wide open.
The biggest problem with 3D Sonic games has been that instead of going with what worked and was fun, they've focused on things no one liked and added even more unnecessary elements. Like from Sonic Adventure 1 to 2, they nixed Tails' levels, which were the 2nd best part of the game. And instead of getting rid of the Emerald Hunts, they made them 1/3 of the game.