Is it possible to allow users to leave comments to a document that lives in Sharepoint? I've seen document ratings, but is there a way to allow comments?
If you are using SP2013/2010, there is a feature for what exactly you are looking for.
1) Go to the document library.
2) Select a document(by clicking the check-box).
3) Open the Library tab from the ribbon on the top.
4) Click on the "Tags and Notes" link on the ribbon.
5) Click on the "Note Board" tab on the Popup.
6) Write your comment and click Post.
Yey....you are done.
Word lets people edit files simultaneously (if you don't force a check-out for editing - works on office 2010 and 2013 (haven't tested earlier versions)), and add Comments to specific areas of text using the review tools (and edit the text as well) all linked to the account editing the document.
If you want it all in browser, you can try installing Office Web Apps on another server. The setup is quite simple...