+ excellent graphic design and animations
+ good, fitting music
+ clever upgrade system
+ for a general idea
- double jump is a little buggy, not always works if you jump just after you landed
What I'd change: I would add a tiny invisible ledge at the beginning of each platform or make the guy run a little faster, since after ~30 plays the only cause of my death was hitting the wall after a failed double jump.
Ideas for sequel: add changing the lead color along with the monsters and background after the player reaches certain distance. Also, since you already use the multiple plays mechanic, why not a classic shop/upgrade at the death screen?
Overall, a good game, deserves all the 4.5 star it gets from me ;)