Interesting. Some comments:

1) It doesn't mention what file types it accepts. Since it says "presentations" I'm tempted to think .PPT, but some tools for presentations (like SpeakerDeck.com) take .PDF.

2) Why presentations? Why not documents (e.g. word processing files)? I haven't used Google Docs, but if it doesn't have feedback capability it seems like documents might be a more fruitful niche than presentations.

3) Perhaps related to (2), would it make more sense to highlight the text you want to comment on rather than drawing a box around it (which doesn't work so well if the text wraps to the next line)?