Ever since the new default comment scheme came out last December, there have been scores of people complaining that they've been unable to comment because the button would remain greyed-out and unclickable. Unfortunately, there seems to be no rhyme or reason as to who is affected by this bug so it's been especially hard to fix.
To address this I just hacked up soph's Preview button Greasemonkey script so that you can add an additional, always working comment button to default comment pages. When you install the script it'll look like this: The new button labeled "Click to comment" is always active so you should be able to submit a comment at any time, even if the standard button (the one labeled "Add a comment") stays grey.
To install the script, visit Userscripts.org and click on "Install". So far this script has been tested on Windows with Firefox and on Mac OS X with Firefox. See the instructions on Userscripts.org to learn how to install this script for Firefox, and this article for Chrome, Internet Explorer, and Safari. If you use this script and it works, let me know what browser and OS you use in the comments.
Note: If you're having difficulty using this as a Greasemonkey script or would prefer to use it as a bookmarklet, [read on.]read on. To make a bookmarklet simply make a new & empty bookmark in your browser. Name it Clickable Comments, and for the destination/location/URL paste in the following code:
Then, whenever you get the grey comment button press this bookmarklet and the clickable button will appear next to the inactive one.
Special thanks to Sophie for writing the original script and big props to markf for giving me insight into how the button is supposed to work & figuring out how it was failing.