The animation on opening/closing the tag list thing is jarring. (Having it overlap the contact list, rather than pushing it down, would be much better.)

I disagree, since this would obscure contacts in the list, and since this would also likely cover the scrollbar, make it non-obvious how to view the obscured contacts. Floating things on top of scrollables is something I’d like to avoid if possible.

- Searching for “family” in the text box has no results; that was unexpected.

Noted.

General question: if the whole thing is expected to be implemented in HTML, how will this affect extensions? Would this bit of the UI be unextensible? (Things I can think of off the topic of my head: additional contact data fields such as PGP key; hooks to, say, dial a contact via Skype or something; possibly ability to search via transliteration, such that “Yukiko” will find the first contact in the list.)

Extensibility is something I want Ensemble to support, but I hope to do that with Javascript hook points rather than XUL overlays (much the same way that Jetpack extensions work).

See a more detailed answer here.