There's a general lot I hated about Modern Sonic when Heroes was released. I just never said them till now.
Controls (Moving around)
After Adventure 2 game. The smoothness of controlling Sonic was rather too smooth. Getting him to run in a straight line was nigh immpossible due to analog sticks. While it was compensated by the stages not allowing Sonic to run off most stages easily. The later Modern Sonic games quickly let this go. Going down a straight way now you mostly see Sonic moving lazily towards the left or right. While many don't find this a true issue I do with some of the more up-to-date Sonic games. What's is now called Sonic '06 truly had this problem with mostly Sonic stages as you had to actually be in a perfect straight line at times in order to make it past a certain section of the stage.
A very horribly executed gimmick. Unleashed added this gimmick which at times was rather unneccessary and other times just plain horrible. Regardless of how well you think you're drifting unless done at the right time you're most likely to fly off to into oblivion. Especially while boosting this gimmick just seems non-existant. While drifting wasn't really required on Colors as barely any stage had a section to where you had to do it. Drifting is a rather horrible gimmick and is made for cars not hedgehogs.
Controls (Un-modable controls)
While in the early days the controls didn't need to have options as pretty much one button did everything. But as progress came. More complicated moves came into play. In my case the horrid homing attack being mapped back to the A button while mid-air. Unleashed had this move mapped to the X button while mid-air. This made wonders for me on running through stages as there's no button to mess me up on the 360 while mid-air. Colors however pretty much threw everything back on the A button (or whatever Jump was depending on controller you used). It was easily excused due to Sonic lost a lot of personal actions such as Ring Dash and the like. However I didn't like it.
Difficulty (can't exactly judge but whatever)
After the unforgiving difficulty I liked about Unleashed. Colors comes out with rather easy mode. In what I believe to be one of the easiest Sonic games ever created. Colors from what was shown to me seems to be easy as well. Although the demo only gave us the first stage. I still remember that Green Hill Zone was a lot harder than what this appears. In fact you generally die on the demo cause of stupidity or trying to be a perfectionist. I pray that I am wrong about this judgement cause I wouldn't want to finish this game in 1 sitting to 1 week.
While I do have these gripes about Generations that doesn't mean I outright hate it. (its not other crappy games I've played cough cough.....*****, **:***, ***, etc. I don't have a reason to hate it yet. Also I'll enjoy Classic Sonic more than Modern Sonic.