Taking advantage of the fact that the zClipse project (as well as many others) is hosted by the generous folks at Sourceforge, the ZEF sends a message to the Sourceforge forum id specified for a subscribing project. Because the ZEF uses anonymous postings to forums, there's nothing to sign up for, noone will call, and nothing to monitor save your message thread. And since it only requires a thread subject, users who just want to express thanks or suggest some improvement considered too small for email can do so easily.
If the Zero Effort Feedback plugin proves useful to projects outside of Sourceforge, expanding and adapting it to other systems (other message boards, email, instant messenging) is a possibility.