First and foremost, it has excellent documentation. Everything is explained clearly on the site, and it's got a great interactive tutorial [2]. Contrast this to everybody (myself included!) moaning about the angular, ember docs etc.
Besides that, it's got deep browser support (all the way back to IE6!), two-way data-binding, plus it's intuitive and simple. In fact, combined with the excellent docs, I'd wager any competent dev could have the gist of knockout and be quite productive within an hour. It is also extremely easy to extend if you wish to plugin external functionality. Which leads to the final point - it plays well with other libraries that you're probably familiar with. e.g. I usually pair with Zepto, jQuery and/or AmplifyJS.
In the interest of being balanced, I must say it does have some bad points. The main thing for me is lack of architectural guidance. Nowhere is it explained the best practices for structuring apps. Nor is there any guidance on unit testing. You can, of course, work out all these things yourself, but why must the same problem be solved over and over? Set some conventions and be done with it! That said, there's a framework of sorts being built on top of knockout called Durandal [3] that aims to solve some of these problems. I've not used it yet, but it looks very neat. Also you'd be remiss not to check out Ryan Niemeyer's website [4] for lots of guidance and useful plugins (he's one of the core contributors I believe)
[1] http://knockoutjs.com/ [2] http://learn.knockoutjs.com/ [3] http://durandaljs.com/ [4] http://www.knockmeout.net/
TodoMVC seems to have them all covered (except the latest one from Facebook): https://github.com/tastejs/todomvc http://todomvc.com/
However, you're not quite right with your assertion above. The docs clearly describe the params that both of those methods can take (remove takes a reference/value, removeAll an array or function or nothing)
http://knockoutjs.com/documentation/observableArrays.html#re...
Whatever the case, I'm happy to see that some portion of the internet is enjoying KO!
I just picked it for a new project. It's very, very clean and nice.
EDIT: good luck with your new project if it's your first with KO. If it is, let me know and I can give you some quick tips to get you on the right track
Having an entire data driven view save loads of time,trust me. You can have complexe nested objects, and you dont have to worry about how the view will be updated. Change a value in the object , and the view updates. And Angular works very well with jquery plugins once you get the directive thing.
I still use Backbone for "brochure" sites and campains though, when i port old Flash sites to HTML5.