Time for a big review!
First I have to say that I'm really impressed with it, there's a lot of potential here, it just need some improvements and will be near perfection.
Physics / Gameplay
The Worlds engine was very well used, it's almost unnoticeable that it is being used, except for the big title card in the beginning. I loved how you revamped the idea of the golden shield from 3D Blast, this is how homming attack should be done in a 2D Sonic game. For some reason, spindash don't seem to match like the old games, it seems slower to me. Also sometimes I can get stuck in walls (act 1, the pipe in the wall), and items get stuck in walls too, like a 10 Ring Box in act 1. Except for that, everything is ok for me.
Graphics / Level Design
The sprites are just great! And I like the Triple Trouble feel that the entire game has, but I thought the foreground was too simplistic. But it was said in the posts before and I've seen that you're looking for help in this point, so I don't need to criticize it. My only big concern is about the level design: It's too linear, and forcing holes to be the only way available - specially those that leads you directly into water - is not cool. The other parts has lots of variety and this is good, Knuckles having his own paths is also very good, but anyway I think the whole level design should be rethought.
Music
The music is freaking awesome! There's nothing else to say, awesome is the right word. About sound effects, I just think the spindash sound could vary the pitch like in the genesis games, but that's not like the game is ruined because of it.
Overall
With some improvements, the right graphics, the same sprites, the same music, and the same creative minds working in this project this can be one of the best fangames ever. Can't wait to see more progress!