This is kind of broad, but seems worthwhile to outline a few key areas. If anything better documentation would be nice if some of the below are already addressed.
General problem areas for people relatively new to ember and ember cli
The stack trace
This is really horrendous. And is even worse with modules. Coming from Ruby, Javascript is really, really painful to debug when something goes wrong.
If there are any better hooks, add-ons or specialized builds of the ember js framework we should document this very clearly in the docs. Tips, tricks and common setup guidance always good things to add to documentation.
Chrome debugger performance
I not sure this is strictly an ember cli problem or what the exact cause is. But the chrome debugger seems increasingly unreliable and sluggish. Not sure if it is all the eval’d ember source. But I find myself having to kill chrome and start over repeatedly. And there is a noticeable lag for a debugger statement to show source.
If there are good tricks or recommendations on how to configure chrome those should be documented.
source maps
I think this is related and could improve the debugging story I believe. Currently documentation doesn’t mention of source maps yay or nay (AFAIK). Should probably state that there is no support with brief statement of what it takes to get there. Sounds like we are waiting for it to land in es6 modules see https://github.com/stefanpenner/ember-cli/issues/926
debugging guide
This isn’t really an ember cli problem per se, but it would be nice if the docs linked to or referenced some well researched and organized debugging guides. Maybe some of the seasoned ember pros could spend some time showcasing specific debugging techniques. Really awesome would be follow along guides where a user could check out a “broken” repo or branch and then watch or read how to find the issue using modern debugging tools in chrome. I know there are some true gems out their like @mixonic presentation at ember conf. But if you are not fanatical about ember you might not know about them.
Added bonus, would be some stuff to ease the pain of node ecosystem. I know coming from ruby/rails the node environment is kind of a weird for the uninitiated. And I suspect there are a lot of ember developers for whom ember cli is their first brush with node and npm.
Where to go for further help or research the problem
When I run into seemingly arcane problems I find there is a lot of effort to go search where the source actually is. Probably need better discoverability of the whole ember API. And for the novice a high level overivew/guide would be a nice edition to documentation. Talk about the RSVP and promises, router, resolver, handlebars, add-ons. Describe what these micro libraries are and where to go for further reading. Even a simple index of the “parts of the ember ecosystem”.
The challenge is that the parts are diffuse some of it lives at so many different locations
- https://github.com/emberjs
- https://github.com/tildeio
- https://github.com/stephanpenner
- and many others…
Such is open source I suppose but it would be nice if there was more cohesion to help understand the Gestalt of the ember ecosystem. We can’t all be super geniuses like @rwjblue 
some of us need all the help we can get!