The biggest thing they fixed, was the level design. There's a reason Unleashed in daytime was even worse than nighttime. Gameplay was better, but level design was worse, to the point of seeming like a 9-year old fiddled with a level design to figure out how damn unintuitive and entirely based on trial-and-error levels could be.
In Colors, that's fixed. First of all, you actually start walking before running in levels, having an increasing and decreasing speed as you go, not sliding your feet through every level as soon as you slow down, like in Unleashed. The dodge manuever was placed in an easier-to-find button and dodge sections are pre-warned beforehand. Not only that, failure from dodge sections simply bring you to a lower section of the level, something which is true to original Sonic games. The homing attack performs by pressing jump twice instead of jump and X/Square, which makes no sense. You also can't use the homing attack freely, just for enemies, and the double jump is way more handy than one would imagine. Water sections are nice, they even added a hard-to-get Super Sonic for regular levels and not even for the endgame, and they made the stage design really colorful and fun. Not just that, for a Wii game, it looks very good.
Now hopefully, we'll get all of that, PLUS the beautiful hedgehog engine which renders high definition levels at high speed, and the old-style levels, which perform pretty damn well with the enemies having their old-style animations intact.