This is really good! I agree with the others, that shield idea is pretty unique, and goodness knows, that's a hard thing to do these days. I was immediately like "Oooo, wanna try this!" A little music would add to this, but totally get it, it's a game jam, can't get everything you want in a month :). However, there are no licensing problems with me throwing some John Williams on as I play xD.
Only thing I'd mention is the jump control. It seemed to me that if you didn't get right off that spacebar, the double jump triggered automatically, and a bit before the peak of the 1st jump.
Good stuff!