i wish i could improvise like this. i've composed a few pieces, but they never get beyond a page or two. improvising, never. my bro used to do this and i'd just marvel. whatever he heard in his head - he'd just do. this is probably the most musical indication of people. but, hey- i sightread well. plagarizing off other people's pages of music and just enjoying the creative process vicariously.
i think once you play enough of other people's music - you get the idea for how to put a piece together halfway decent. it's just - i have a lot of unfinished works around.
songs are easy for me though. i can compose decent songs that are short and sweet. once i tried a choral piece. it was sort of a take-off on beethoven's choral at the end of the ninth symphony - but was fugal to start. also, i composed a song about a woman sweeping snow off her driveway (instead of shoveling it). it was a sort of 'modern' sounding piece and had some large leaps in it that made it sound 20th century - but still had a nice sound. kind of like samuel barber's stuff. or ives. sort of modal.
now, this was kinda modal, too - right? how many notes did you stick with?