I downloaded this to take a look at it.
What you have got is a nice interface for quicktime instruments and other banks that could possibly be loaded and the ability to use the mac's keyboard as the input device. The input thing was something I wanted a while back, and I was pointed toward midi keys.
The thing this little app lacks is variety. It is taking sounds that pretty much already exist and allows you to play them and add a little reverb (which is nicely done).
A large commerical app can have a lot of settings, controls and knobs where you can completely change the tone or sound of a preset and basically create your own sounds.
I downloaded microSynth from a really cool link.
Check out some of the other synths out there in the list at
http://www.hitsquad.com/smm/mac/SOFTWARE_SYNTHESIZERS/
Check out crystal, bass station and slayer as 3 examples of what can be done.
These are actually available as plugins which work inside of metro.
Microsynth seems to be its own app.
I guess the best way to describe it would be a metro midi track would give you the note, length and when to play it and the virtual instruments give you what they're going to sound like.
scoot.