Um no, Mario didnt change one bit, he just had a very small evolution a real one at that making things better and better without changing him. While Sonic had a freaking whole character redesign which is hardly even recognizable. You look at the 90's boxart, sprites, cartoons, now that's Sonic the Hedgehog, and then you look at the "modern" look now that doesnt even look like Sonic.
Sonic doesn't have green eyes, buckles, skinny body, long legs, extra random squid spikes, and so on. This modern thing is like a fan character. Sonic was made to look and act like an American kid of the 90's.
Hey Matt, that pattern is awesome for the background! I love the alternating fade, and great choice for color holding all the action lines. I think the light color drops out the speedlines for the stars closest to the background characters, and improves the energy of the piece by giving the further ones exaggerated movement. Good choices! The only thing I personally want to see pop out a bit more is the shine effect on the emerald.
From what I've noticed, Sonic's arms and Knuckles' left arm are too long, and I question the location and placement of Eggman's right leg. That said, the facial expressions are magnificent, and overall the cover is popping with energy. A pretty good cover, but with some tweaks it could be greater. I hope Mr. Wells can take constructive criticism (unlike some other popular artists on DA), as I believe he has the potential to be a truly wonderful artist.