I just gave it a try and didn't like it much at all. Now, I'm not a hardcore Sonic fanboy, but I did play Sonic 1-3 (more 1 than anything else) back in the day.
1) The walking animations really bug me. As you speed ramps up, Sonic uses 3-4 different animation. So as your speed ramps up from a dead stop, he starts to look like he's gliding across the ground. The only time his animation really looks good, is when he's going fast enough for his legs to be a blur.
2) When Sonic jumps, and becomes a ball, he's lopsided. His hair sticks out just enough for him to be lopsided and it's a little distracting.
3) When Sonic is rolling and manages to become airborne, he comes out of his ball shape, making him vulnerable to enemies. In the first level of Sonic 1 there's a straight section with a short quarter pipe incline to run up, followed by a bridge with 2 fish jumping up. You can roll into a ball as you approach the incline, and when you come down onto the bridge, you're in a ball so you can land on one of the fish. In Sonic 4, the same exact geometry is in the level, but landing on that fish hurts because you can't stay in a ball for it.
4) You have to hold in the direction you're moving to maintain momentum. This may seem a little nitpicky, but this is not the way the originals worked. I got into a section where you hit a speed boost, then go through a big S curve, followed by a loop that you enter from below, and a vertical ramp after that. I did not make up up the ramp because I didn't maintain my momentum. So I went backwards to try again, and had trouble figuring out exactly which direction to hold while going through the loop. Do I press in the direction he's heading? I tried this but was too slow and made him skid to a stop. Do I just hold to the right? I have no idea.
Maybe these weird things were in some of the later Sonic games, but I missed them because I only played Sonic 1-3, but they didn't do a very job of replicating the gameplay of 1-3, which is what it seems like they were after.
