The conversion process within YouTube does affect the sound quality of the uploaded videos to some extent. As far as uploaded quality is concerned, Vimeo has a higher standard to YouTube.

Janie, how are you doing the actual recording? Do you record directly into the Mac via Garageband with an external mic? Or use the internal mic? Do you add artificial reverb to your recordings either during or after the process?

My experience with digital reverb is that it's not easy to control in a post-recording environment - as Dave has rightly pointed out. I now never use it.