Some people argue that adding a dynamic commenting system contradicts the static html goal of Jekyll. Going along those lines, critics say that a conversation is best carried out on 3rd party sites like HN. However, for technical blog posts specifically, a two-way conversation helps illuminate finer points in the original blog post.
Googling around for 'jekyll comments' or 'disqus open source alternative' reveals many people who would use such an idea. However, aside from some quick-and-dirty projects, I see no compelling open source alternative to disqus that will include features such as OpenId, spam prevention, comment threads, etc. Does anyone know of such a project?