OK so I've played through The Fairy Glade on PC and if memory serves, I can tell you that there's at least 3 less Robo-pirates in the level VS the Dreamcast version, along with one less laserbeam as you exit part 3.
Looks like there's going to be a lot more differences than I expected.
Edit: I never had a problem getting through The Marshes Of Awakening, but I never even knew there was actually a button to make Ssssam go faster, despite playing and completing the Dreamcast version more than once, along with other platforms too.
Edit 2: Another difference is the way Rayman walks down slopes. On the Dreamcast, he moves his feet really fast just like someone would in real life. On the PC, he just moves his feet at the same pace as normal.
Another difference on PC vs Dreamcast, is that The Cave Of Bad Dreams doesn't have rotating platforms on the Jano fight and after you get the Elixer Of Life, the level doesn't just automatically end.