I just gave this a shot and to avoid past criticism here is my own :
First off, Rocky Falls Zone was great. The worst part is Rocky Falls Zone 2's beginning. Starting me off in water, not to mention dark water, is not a good way to play as Sonic. If it wasn't for the incredibly large amount of air bubbles around, I probably would have game overed right then and there. It was next to impossible to figure out what I had to do, because there was no indication and the water was so dark I couldn't see any springs. This is also a problem later on when I got trapped in a very shallow river. I eventually ran into a blue spring by running along the edge of the river.
I got lost in almost every level very frequently, whether it was trying to find the single spring you put that enables me to proceed, or the rooms were so dark that I couldn't even see that death was directly below me. I really how you make the darkened areas a little brighter.
On your 2D sections, I thought the conveyor belt idea was pretty neat, but took me a second to get use to. However, Twisted Temple's final 2D area is not very forgiving. There is one flame that is near impossible for Sonic to jump over, and the part with the bottomless pit near the end gives no indication there are chains to grab onto. I took a thok of faith and luckily grabbed onto one, but I suggest adding one more pillar so people don't get confused and think it's just a thok of faith area.
Overall, I find that not enough testing with Sonic was done on these levels, as some areas are near impossible for Sonic. Please work on this.
Also, Tails' path in Royal Palace Zone 2 is a lot easier with Sonic than taking Sonic's path with Sonic.