Quick response to this. Well, a couple of responses.
First, people keep thinking information management is easy or should be easy, but the truth is, it just isn’t, at least once you get past the wedding guest list stage (and to be honest, even that can present challenges). Understanding what types of data you’re dealing with and how best to relate them in the context of the real-life problems you’re trying to solve, involves a combination of pretty abstract thinking with close attention to concrete details and it’s a tricky balancing act. Most people aren’t good at the abstraction – this is perhaps especially true of people who work in the industry, because they really do know the trees (and the rivers and the hills and valleys) so intimately. On the other hand, I know a lot of software developers who are terrific at the abstraction and never quite figure out how to create something that is actually useful. And even when the data modeling is pretty simple, their are often challenges with workflow, user-interface design, and so on. It’s a bit like photography (something I’ve done quite a bit of). A lot of amateurs think the hard part of photography is figuring out what camera to buy. 
Second, I think Coda’s documentation is still pretty limited. Lots of nice videos that explain how to do very basic stuff, but not much on how to build more complicated documents. I am confident that as Coda gets a little more established there will be more help available from various sources. (I’m working on something myself.)
Third and last, learning a tool like Coda is always a circular, inefficient process. I’ve known a fair number of what are sometimes called “knowledge workers” (lawyers, scientists, etc) who get the idea they need an app to solve some problem in their work, and they think the tool they find should allow them to go right to the solution they seek, as if they were using Google Maps to get to Milwaukee. The truth is–and this is a bit Zen, I think–you can’t go straight to the solution you want, at least no beginner can. Instead, you have to spend a fair amount of time aimlessly learning about functions and features you don’t see an immediate use for. Until you have the vocabulary provided by the tool to solve the problem you face, you won’t be able to understand the problem. In my consulting practice I see this all the time. Clients hire me because they think they need X. We meet, we talk, I learn about their business, and in the end we learn together that what they really need is something more like Y with perhaps a bit of Z.
That said, I’m finding Coda pretty darned impressive.
William
p.s. Thanks to you or whoever it was that mentioned Fibery. Hadn’t heard of it before. Looks promising – but has a good ways to go yet.