(Damn, typed in a wall of text only to lose it in an auth error).
My post sounds pretty negative, since all the positive stuff I had to say went to the chat itself, so here's a summary:
What I use the most is logging (not to be confused with IRC nick identification. Lucid, I'm looking at ya!) and /whois and /whowas (so that I can bite the heads off impostors). Everything else is customization, like notifications: I'm only notified when someone mentions me, and not when someone just says something. Then there's a ton of small things like a multiline input field, not sending on Enter, etc. All in all, it'd take a lot of work to match the functionality of a standalone IRC client.
Tom mentioned setting up Scirra's own IRC server, and that is a really great idea. One one hand, we get forum integration, on the other, people with personalized IRC clients can still use them just fine. Win/win.
Edit: Oh yeah, another thing I use: being logged in from several PCs at once. All the better to see with, my child. >:3 Of course, on IRC I have to have several usernames for that to work. Shviller2011-07-13 21:56:13