Livefyre Sidenotes plugin for WordPress does exactly this:
It enables contextual inline commenting not only per paragraph, but, within this, commenting on a particular section highlighted by the user, which would be closer to the MS Word implementation:
Start by highlighting the text:
When the comment has been posted, the highlighted selection is shown upon hovering over the comment posted:
I'd also been looking for a plugin to do this for quite some time, for an articles discussion site. I had come across a few, but held off implementing one due to uncertainties regarding possible error or incompatibility either now or in the future. Although there is a downside from the point of view of data portability (presumably, the comments would be tied down to the Livefyre service), I would personally go with this solution, given that a company devoted to commenting systems is behind it as a guarantor and support for the foreseeable future.