You're right, I'm really on the fence with this, it's a hard one. I'm going to look into getting a light-weight HTML email for the system notifications. If the right balance can be struck I'll go for HTML.
Edit: When I say HTML I mean well designed (boxes, colors and logos). At the moment, like tomwalsham said, its just some <p> tags.
Just to add my voice to the thread above—I also thought it was odd to get a plain-jane invite email. The overall idea of the app is really enticing to me though because I really hate coding up emails using the old-school tables, font-face, etc.
Edit: When I say HTML I mean well designed (boxes, colors and logos). At the moment, like tomwalsham said, its just some <p> tags.