As you might expect, each Robot Master has his own unique weapon and abilities, and the stages have been arranged to take advantage of those skill sets. At the outset, you must play each stage with a certain Robot Master. When it's cleared, however, that Robot Master becomes unlocked and free to use any time; you can even switch Robot Masters in mid stage.
I only sampled it a small bit so far, but I was really taken by the new elements. Stages definitely get longer and more challenging, but are never crushingly unfair (maybe a couple cheap enemy placements, though). Truth be told, though, I'm awful at playing platformers with a keyboard.
While the above trailer cites the game as 0.6 beta, the completed version 1.0 was just recently released, and you can grab it here. If you like trying out Mega Man fan games, I definitely recommend it! After all, you can play as Heat Man!
Thanks for passing this along, Rynen10K!