Comment voting appears to be nonfunctional (the little up arrow does not appear at all) when using Internet Explorer 10 on Windows 8. I have not tried other browsers.
It appears specific to my Windows 8 machine. IE10 lets me upvote comments on my XP, Vista, and 7 machines. Yes, I own a menagerie.
Update, with more details in order to reproduce I have been able to reproduce the problem. I have a Lenovo Thinkpad Twist. I can upvote the comments when I am using the Twist in "laptop/notebook" mode with the various mouse replacements, like the touch pad and that annoying little nubbin in the center of my key board. I cannot upvote comments when in tablet mode using either my fingers or a stylus. This seems to be a touch screen specific issue.
I can vote on posts just fine in either mode.