Have you ever missed an alternative to the like button in Facebook? Many times I find neither like or not doing anything reflect my view. Comment voting introduce a more finetuned valuation, which I welcome. A post from ReadWriteWeb by Mike Melanson tells more about the new feature:
In the new comment plugin, comments are not only threaded, meaning each user can reply directly to another user, but more information is shown on each person, including their job and company, or network, and their comment record. The system also allows for up- and down-voting, a la Reddit – actually, the whole comment system is very Reddit-esque.