Graphically, there's a few issues as I see. However, I'm sure that since this is just the beta that a lot of this will be remedied.
•The grass (or moss?) is looking much more dull next to the original, primarily because of the less-saturated tones you used. I don't mind the color, but you just have light and dark, and I think it would do well to have a highlight color added.
•I really like the background, specifically the smaller trees. However, the tall ones with the gradient seem out of place. Also, the blur on the furthest layer of trees seems very immediate and sort of cheap... I would advise removing it, or making it a little less obvious.
•There's no parallax scrolling in the background. This really sticks out and I think that the overall presentation would be extremely improved if it becomes utilized.
•The flippy-vines. They just look cheaply made with gradients, and I echo the previous statement of switching up the original art style of them and going with a look that's different. It would really help!
•Sonic's model. Like the grass (and everything else here, for that matter) could use highlights. You have the regular color and the shadows, but no highlights and for sake of contrast, it would improve the look overall.
Generally, I would say you should avoid gradients, but there are ways to make them work. They're not really working here, and I would encourage you to look into alternate shading techniques and come back to gradients if no others suit your needs. The way they're being utilized is a bit blatant and it sticks out like a sore thumb, undermining the up-scaling you've done in general.
In summary, less gradients, more highlights, nix or reduce the distance blur. Additionally, hoping the framerate will be improved, and more frames added to Sonic's animation. I'll keep an eye on this, as I'll be curious to see how this shapes up.
