The only version of the original Donkey Kong Country that I ever played extensively, was the GameBoy Colour release. Now I understand that version of the game had some differences from the Super Nintendo version. Obviously this new game was designed as a throwback to the SNES version, not the GameBoy one. My question is this: How different exactly were the SNES and GameBoy versions of DKC and will I be able to appreciate Returns since it is a throwback to the version that I didn't really play.

I think they were practically identical, gameplay-wise. Same levels and everything.

According to GB's wiki, the GBC version of DKC was basically the same with some added mini-games. So yeah, it'll be like that.

They tried to emulator DKC on GBC the best they can. So it is what it is. Obviously, the SNES version had way better graphics, animation, and soundtrack to it. Gameplay-wise, they kept it pretty similar though. Level layout remained the same pretty much as well too, from what I remember.
 
So that being said, your DKC memories may not be as vivid as everyone else's (as someone who played SNES DKC1-3), but I think you will be able to enjoy DKCR just fine. It's a reboot, so they fully expect new players who've never heard of Donkey Kong to come in and enjoy the title.