Well milkshake, it sounds like you're already contemplating the next version of the repo spec. I don't think we should be making any changes quite yet; we probably want to wait to see where problems appear first. And even if we do decide on changes, don't go implementing them until we've formalized them.
Having said that, we can certainly start brainstorming!
A datestamp could be useful to save on bandwidth and processing. I imagine this would be something like a "last-updated" field within the "repository" field. Problem is that I could use this to quickly determine if there have been no updates, but I can't think of how I would robustly implement something that gets only as many bytes as necessary and then parses it without having the full JSON page there. I'm sure it could be done, but I'm not sure all the hacking would be worthwhile; after all, the repository page shouldn't be all that large (yours is only 68kB so far). Then again, I'm not very well versed in web technologies, so maybe this would be easier than I'm making it out to be. As an alternative, isn't there some way for a server to say "this page hasn't changed since you last requested it"?
App ratings could be good; my only concern would be that different sites would implement different ways of rating software. Perhaps the spec could include an optional "rating" field for each app that's given as a percentage; then, any spec-implementing site would just have to translate their own rating system into a percentage, and any client could display this percentage however it wants. Now, when comparing with traditional package managers, a rating seems a little superfluous; however, when comparing to commercial app stores, it doesn't look like a bad idea.
Comments trouble me, simply because how many would be included? how big would the repo page become? Perhaps comments could be handled like icons: if available, a URL to download them (also in JSON form?) would be included for each app. Again, though, this seems superfluous for a package manager, and possibly even for the client-side of an app store. Perhaps, instead of including comments, each app could include a link to its human-viewable page on your site; if a user wants to see more about an app than is available in the repo, the client could launch a web browser and open that page, whereupon comments (and any other extras, like screenshots) would be viewable.
I wrote too much. Keep up the good work, and I'll get that client caught up to you soon eventually.