I hope Dawid and Oskar find somebody to help them with this experiment and I hope encouragement and constructive feedback start float to the top of this thread. Lets help these guys get help and see what happens!
I'm not saying that it's impossible to receive some good design advice on HN as there are certainly some experienced design-based folks dotted around, but on the whole it's a bit like asking for database optimization advice on Dribbble.
This wasn't an invitation to join their project or even any mention that they would spend any time on it themselves. Hell, they didn't even include the source graphics to start anyone off.
So...what's the problem?
They want to fork the project and build a new UI? Cool. Go do it. If people like their changes they'll start using their version of Adium, and other people will contribute to their project.
It's not just an idea. It's a considered UI for the community to critique and possibly implement. An open source project thrives on multiple people to help out. If this mockup was so simple and obvious, it would have existed already.
To follow the analogy, one could use the Cowboys stadium as a precedent and then adapt it to the climate of Minneapolis.
This isn't a discussion about the codebase because the code can't solve the questions being discussed. There isn't anything in the concept that is out of the realm of feasibility. Critiques happen at all phase of design and development, and you seem like someone who's uncomfortable with ideas that aren't finalized.
Anybody can make something pretty in Photoshop. If he was serious about getting people to implement his mockup he should have spent a few days writing a quick and dirty implementation, with real code that could be improved upon later.
What's interesting is the interface, which is why we're discussing it. The fact that you can't run an application doesn't make the concept any more or less interesting to discuss and critique. The issues being discussed aren't solved with code.
You know that there's a profession called UI Designer?
If I had the time and knowledge of the Adium code base I'd be thrilled to collaborate with these two talented individuals. Hopefully there are developers out there with open enough minds, time, and passion to work with these two.
The fact that something looks "early 2000s" is so far down the list of my priorities for an IM app that it's effectively a non-issue. I think a lot of the sentiment here is simply that the important issues aren't really being addressed, and implementing a radical change like this is likely to raise a whole slew of new ones.
If Dawid and Oskar want to progress from mockup to making this a project, I wholeheartedly encourage them visit http://www.adiumxtras.com/ and learn how to reskin Adium. Their talents clearly show that if they put even a minimal amount of time into it, they would be able to accomplish most of the UI on their own.
Adium is fine as is, you can customise the hell out of it, it's tab implementation is solid. The only places it's lacking are more server issues, file transfer hasn't worked in years for me.
The concept of "staleness" encourages change for the sake of change, and I don't get it. Please ask non-designers if they want the look of their favourite software to be re-imagined.
The feedback to this is predictably similar to the transition from Skype 2.8 to Skype 5 (OS X), and I know people who still cling to their Skype 2.8 binary.
Average consumers will choose a nicely designed app over one that looks dated, especially if they offer the same functionality.
Edit: I just realised that I have hidden the ugly message window toolbar. I'm not sure if it aged though - I already hated it on OS X 10.4 (when I disabled and then forgot about it).