If you suddenly can't see comments on some of your favorite blogs, there's a very irritating reason for that.
It seems that in their most recent update, the AdBlockPlus filterset has started blocking all embedded content from JS-KIT, who serves Echo and Haloscan.
Looking through my logs shows it might have something to do with a specificclick third party cross site reference.
Several other plugins use the same dataset for their filters. If you're using one of these filters, you're going to need to add an exception to your filterset to be able to comment, or view comments, on any of the perhaps 1/4 of all blogs (or at least all active blogs, that are regularly updated. Most less active blogs just use the default wordpress or blogger comments) that use either Haloscan or Echo.
Just a guess, but I'd say this is because of all the antisocial asses out there (mostly linux users, and almost entirely firefox users -I use both, but I'm not one of the aforementioned asses) who report any java, script, or tracking cookie as spam or ads to their filtering services.
Meanwhile, here's a video tutorial on adding exceptions to AdBlock Plus:
UPDATE: Well, that was quick. Either JS-KIT updated or the filterset did, I'm not sure which.