MochiKit has HUNDREDS of tests. We build real applications with this thing. So even though development can move fast, we make sure to get tests written. This also makes platform compatibility issues much easier to detect and resolve than the "guess and check" style of quality assurance seen in some of the other libraries out there. It's not broken.
MochiKit can adapt to anything you throw at it. It makes no assumptions about how your code needs to act, and it has hooks (by way of the the adapter registries) that makes sure that you can define your own comparisons, programmer representations, iterators, or DOM node coercion for any object in any way you wish. We'll gladly serve you all the Kool-Aid you want, but we're not going to make you drink any.
Plays well with others
As far as server-side goes, MochiKit is totally agnostic. However, feel free to include MochiKit as part of your AJAX arsenal - TurboGears does out of the box!
Free as in…
You won't see any political agenda nonsense here. MochiKit is available to you under the extremely liberal MIT license, or the written-by-a-very-smart-lawyer Academic Free License, v2.1. It's your choice! We don't want to "infect" YOUR software with our ideals, we just want to make every developers' life suck just a little bit less. MochiKit is free as in we don't care what you do with it. With the choice of these two licenses, you can put MochiKit anywhere. We certainly won't refuse if you offer free beer, though.
Yes, we're aware that a lot of people use other browsers, but the above list matches an astronomically high percentage of the intended audience of our flagship application. Please let us know if you encounter any problems elsewhere, especially if you can contribute a fix.