You are right, listserv’s have an amazing number of participants (experts), a question will certainly fall on a number of ears.

But listservs aren’t a well organised knowledge base, what I like about Q&A services is that some allow you to tag yourself, so you can find the appropriate expert, or you could just ask a group of experts eg. FAQQLY groups, or ask the whole service like Otavo.

Viewing people organised by tag, and past and present content organised by tag, allows you to perhaps lessen the annoyance by asking the right person a question, or finding the answer in the knowledge base without having to ask a question.

Sure you can tag anything in del.icio.us, but the new services these days like to specialise in a topic area as bookmark managers are general…how do you find the Q&A bookmarks apart from the other general bookmarks.
You are right a bookmark service dedicated to just listserv Q&A would help, but then it is not conducive to asking questions, it will only store answers.
NOTE:I suppose Tag-based forums like Vennt could have a special version dedicated to Q&A.

This is why I like the idea of a dedicated (bookmark/tagging) service for just Q&A.
Listserv’s are numerous, and not all of us know where they live or that they exist…I like the idea of a service that specialises in just Q&A, and that you could perhaps form listserv type groups.
(Discover people by tags, discover groups by tag, discover content by tags, view a persons user space…even like Yedda, it will invite you to answer questions).

I know what you allude to in that listserv’s are already set up and have dedicated membership, and building on top of this would be easier than trying to assemble a new community…in the end community is king, a service will only “be” if there is a community to service.