For quite a while now, I've been using a popular Wordpress plugin called "subscribe to comments" on fredericiana. It allows people to request email notifications when further comments come in on an article they commented on themselves. As it turns out though, the plugin has an unclear legal status in Germany [de]. The fact of the matter is, since somebody can put an arbitrary email address into the email field when commenting, a person who didn't want that may get "spammed" by my blog against their will.
That would result in a situation where the seeming "victim" of my "spam" could sue me for unsolicited advertisement, provided my blog is considered commercial--and depending on the judge, even a link to Amazon can make a website "commercial".
It is sad that German laws are so Internet-unfriendly, but I have to live with them. So I switched off email notifications on my blog until further notice.
I may switch to a double-opt-in solution (i.e., before notifications are sent, people have to click a link in an email to confirm they actually requested this service). While this is also not completely safe in legal terms (that means, no significant court has ruled about it yet), it is currently considered state of the art when it comes to sending out any sort of regular email to a group of people.
Since a lot of people are facing this problem, maybe the plugin's author or somebody else will extend the plugin to have double-opt-in functionality soon. I may even do it myself when I am bored the next time.
Until then, sorry for the inconvenience. You can always subscribe to the Comments RSS feed if you want to stay up to date with what's happening here.