* It just works
* Inline image viewing
* You can customize anything via Propane[1]'s custom javascript support.
I don't know about any clients on Linux, but, Colloquy (and most IRC clients for Mac, I suppose) use a WebView from WebKit to display text, so they'll have points 1, 3 and 4 covered. And point 2, if you don't need chat history.
(disclaimer: I contribute to Colloquy for both iOS and Mac.)
Point 2 is actually the most important point for us. Since Tom was the only one that liked irssi, the rest of us lost the backlog and that was a deal-breaker.
(And happy to hear you use Colloquy! :D)
OT: I think convore will go the same way, unless the crew can seriously address the SvN problem.
Anyway, when I worked at an all-remote company, we just ran our own IRC server. Then all the channels were work-related. Of course, it was pretty easy to also join other networks, which we did (but would do anyway).