Makes sense ;) Was there any reason why you went with Cappuccino instead of, say, SproutCore or Coherence or Dashcode? Or even GWT? Have you looked at Apple's PastryKit/iAD? It's _very_ similar to Cocoa development, just without the ObjC notation.
I spent a lot of time looking at different frameworks, but in the end, it's pretty hard to judge them without diving in and actually making an app in it. So I have some knowledge of the Cappuccino competitors from my initial research, but obviously nothing as in-depth as my knowledge of Cappuccino, so take anything I say about the other frameworks bearing that in mind. Also this initial research was done around May of 2009.
The main selling points that drew me to Cappuccino at the time were 1) there was a live app using it (280Slides) that demonstrated doing a lot of the things I wanted to do with Mockingbird, 2) Even if there weren't tons of docs at the time, Cocoa documentation is very extensive and applicable, 3) Because it pretty much implements the Cocoa API, I knew I was getting an API that had been used successfully in tons of applications, 4) The promise of Atlas, 5) it made my app decent looking without my having to try very hard (though my co-founder Sheena did end up customizing the Mockingbird look and feel a lot), and most importantly (at least when picking a framework when you don't have time to get more than superficial knowledge of it) - 6) after spending some time in the IRC channel/asking a few questions on the mailing list, the creators and other members of the community were extremely helpful and welcoming, the 280North guys seemed to have a very good sense of where they wanted the framework to go and ideas for awesome features they wanted to implement, and because they were such good developers, I was pretty well convinced that they would actually ship their promises (and they have - Atlas, automatic mage spriting and app flattening options were all things that didn't really exist when I picked Cappuccino).
As for the other frameworks (and there were a ton, but I'll just talk about the ones you mention):
GWT - I didn't like the compile step, and I've done some Java and I don't prefer it. Also, bigger communities like GWT's have their pros and cons - you probably have more resources you can Google for, but it's not always as easy to just ping one of the core devs and get personalized help/get him to help you write your patch.
SproutCore - probably the closest competitor to Cappuccino back then. In this case, Atlas was a selling point for Cappuccino, and I thought Cappuccino apps looked nicer by default. I also don't remember seeing any impressive SproutCore apps in 2009 other than MobileMe, but I might be misremembering.
PastryKit didn't exist in the summer of 2009.
I believe Dashcode emerged as more of a web application creator with version 3.0, which was fairly recent. If not, I may have simply not looked much into it.
