Firstly, I'll be honest, the review is over generous for just how polished this is: the graphical style, the environments, the controls etc all amount to a very proffessional, complete feel. Managed to completely avoid feeling like an internet game.
However, my main criticism would be the actual game-play itself. Really, it just amounts to arsing about collecting pills, with a lot a backtracking (in the levels themselves and playing levels again) and the enemies are most often sparse but often irritating to try and hit. I guess I'll draw the inevitable comparison and point out in the likes of Sonic, the ultimate goal doesn't feel like collecting as many rings as possible, but soaring through the levels and it's many obstacles, without too much frustrating grind. In that respect then, this lacked many obstacles other than elaborate layouts and trying not the screw up the order you collect stuff in (loosing access to parts of the level etc). In essence, I feel like this was a bit lacking in an fun game mechanic. There were also some instances of incomplete level design, but that doesn't really mean too much.
However, as a whole, the game was pleasing enough to make me want to (with some annoyance) collect everything, and played well enough to accommodate for the slight repetitiveness. I would love to see another release with more enemies, slightly easier combat (bigger hit radiuses comes to mind) and more bosses, and then you would certainly be with the same realm as the original Sonic releases. On that note, I loved the sense of progression felt as you went from zone to zone, and the use of elaborate parallax backgrounds to form the settings, it adds depth and I think it's something Sonic and Knuckles did particularly well. Also, kudos on all the creativety: the pixel art and animations, this game was definitely an aesthetic trumphiant!
PS just cos I didn't enjoy the gameplay wholly, doesn't men others didn't too, just my opinion! Again, a very very polished release, the most i've seen on newgrounds, would love to see some kind of Xbox arcade/ steam style release!