I'm not much of an improvisation critiquer, but I'll try to give a shot anyways.
You're a lot better than I am. Period. Yeah, I play piano. No, I can't improvise worth crap. I wouldn't call this a mastery of chords and melodies, but it's a damn sight better than anything I could hope to do. Well done.
I think that in many parts of the song, especially in the first half, you could've moved your left hand down an octave or so. It seemed a bit too high-pitched for a left hand, although the right hand was pretty much fine. There are places like 0:54 when the different registers played with your hands work wonderfully together, but I think they're a bit too close for too much of the song.
Also, the echo seems like it's a bit much. Personally, I'd prefer it with a bit less echo and more reverb, but that's just a personal preference, I guess. Speaking of personal preferences, the only reason I'm knocking it down half a star is because I'm a tempofreak. I'm a drummer, so it really annoys me when you have a song that's going along at a steady tempo and then some melody starts going on that misses the beat by a tad. For improvs, you sort of go in expecting to not hear the steadiest composing, but I think you could've done a bit better. Then again, it was late night. We all think different when we're tired. :)
Nitpicks aside, this was a wonderful piece. The ambience was done well and the melodies were great as usual. I couldn't really hear any glaring errors notewise (e.g. cracked notes, 'semitoned' notes, etc.), so it seems you have good control over where your fingers go, but not as much as to when to make them go (tempo!). Great job, and I'd like to see more of this stuff from you. It's a great addition to the chiptune break you've been having.