Microsoft today detailed the upcoming commenting system in Office Web Apps. You can now share your document from your desktop on SkyDrive and allow anyone to add comments directly to the document. They just need a browser to do that and all the comments people make are stored along with the document. So when you download it to your desktop again and open it, all your comments will be right there. You can now even reply to any comments.
Read more about it at the source link below. You can try out this stuff by joining the preview.
Source: Office Blog