After looking at the mockups and reading some of the responses from the devel mailing list, I've got a few comments. Apologies for the long email and some of it being addressed already or being preachy.
This is reminiscent (heavily so) of Skype's 5.x ui. There's a wealth of complaints online about the usability problems with Skype's single window design, notably that it's confusing and makes it incredibly difficult to just tell someone (verbally or via text messaging of some form) what to do and often to actually know what to do until you've figured it out by random guesswork. It's an interesting idea but it breaks almost every commonly accepted chat client's ui and expected norms. The exception I can think of is irc clients and, in general terms, irc isn't for the average user and first time irc users tend to be very confused about the interfaces for a while. The mockup addresses some elements of the issues with Skype's single window design. It separates the contact list from the conversation/chat list with a search bar, it uses distinct ui styling for contacts in the list vs the contact chat name area for differentiation. Issues to think about are things like: closing chats versus closing the entire window via hotkey or even via the stoplights in the corner (effect on connections to irc?) is it actually easier to combine your tab list for conversations/chats/whatever with the actual contact list? Is that actually easier for people to use/explain to new users? How will that affect lists of people in the rooms for group chats/irc? what about the people who adamantly don't want to use tabbed chats? or swear by using tabs for conversations in some location other than the left side of the window? what about people who have use cases where they want a minimally sized chat window or to view all of a large contact list? looking at the mockups, I'm uncertain if I can open a chat with someone by double-clicking them off the contact list because it opens a contact card? looking at some of the options at the bottom of the pdf, where it shows collapsed areas makes me wonder how obvious/non-obvious it'll be to people how to fix that if they press the button. how obvious is it to the user that a new chat has been created without someone telling them how to do it? Most users grasp "double click the name and type in the new window that appears". Skype's new 5.x design is a big offender on that score because although it does create a chat "tab", it's not a big change that instantly makes people go "oh new window, I type here!" and there's no widgets informing you that there's a new tab, just a line of text in the sidebar. I've been writing this email for long enough that Evan (among others) has mentioned a number of points/options that respond to some of my concerns. The last few emails have largely seemed to suggest that most of you who've posted would not be happy using adium as presented in the first page mockup without tweaking the appearance or the behavior, often in order to bring elements of it in line with the existing ui and functionality. For example, Evan has just suggested that closing all chat "tabs" should revert to just a contact list window. People will tend to use what they get shown first, at least for a while. If your response is ever anything but "the defaults are perfectly usable, and I can use it that way" you need to rethink what you're doing and what the defaults actually are. If they are so unacceptable that many people must change them to use adium, that suggests the defaults are so unacceptable that a lot of people won't like adium and won't continue to use it. Also worth noting, adium has an extensive existing user base, how much will need to be redone wholesale for this and break existing user habits? How many existing users will go elsewhere because "x useful behavior isn't there/the same"? and how important is that question to you?
