Jeremy, what I've seen most people struggle with (including myself in the beginning) is those first couple of hundred lines of code. How does everything fit together? How should I handle templates? How to I create nested models? How do I stop thinking "in jQuery" and start thinking "in Backbone"?
I guess the thing is that the current documentation works really well when you understand Backbone. Before you understand it, getting started is really difficult. That's why we see so many getting started guides all around — the problem of course being that most of them are barely just ok. Backbone should have a really, really good getting started guide, which (I think) should be more opinionated than Backbone itself — i.e. it shows one (really good) way of doing things (that works well when you scale up your application). Or at least link to one.
I've heard many say "look at the todo example", but I think that it's so far away from a real app that it's difficult to use as a starting point. It doesn't use ajax, it relies heavily on global state, it puts templates in the HTML, lots of global `$` stuff, no routing, etcetc. Ok-ish choices, but not a great example for those getting started. E.g. having templates in the DOM is ok when you have 5 views. When you have more it's problematic, especially for testing and for precompiling templates.
Backbone is great when you grok it, but it takes a lot of time to find your own patterns that works really well. (Marionette and Chaplin are awesome, but still, first when you have some basic understanding of Backbone itself)