Actually, the difference in difficulty between making it slippery and non-slippery was very small.
In some spots, I found the level was actually easier
due to slippery physics, such as any spot where you're try to jump off an uphill slope because you don't immediately begin sliding down when you stop running. Though I guess it does make collecting the contents of those coin blocks in Screen 11 a bit of an annoyance... that's about all I could find wrong with using ice physics here.
I personally thought the palette alone should've been a good indicator this level was icy (along with the fact this level would be placed into the Ice World in the collab ROM), but I made the traversable foreground a bit lighter. Understand I can't make it too light, or else there would be issues with palette contrast, especially once I start adding more decoration.
My Let's Play channel
(Current LP: Final Fantasy VI, MegaMari)