Gameplay-wise, I think they nailed it with Generations. I enjoyed it more so than any sonic game since Heroes. If they had added a Chao Garden into it it would have been so awesome. The garden is really the only thing left I want from them now. If they keep their gameplay in this area when going forward, and add a Chao Garden, they'll have it made.
The only aspects I miss from Adventure/2 was playing Knuckles' levels (searching for the master emerald pieces was fun), and the open-world feel of running/walking to the next level. I can live without those two though.