Great post. I do a lot of these same things whenever I join a new app or am evaluating a project to take on. There's definitely some doozies of Rails apps out there.

I also like to pry open the ApplicationController pretty quickly. You'll usually find all sorts of interesting logic and tight coupling in there.

In the example app you used, the models actually look pretty light on lines of code. I'm willing to bet the Views were full of conditionals, which is another thing I keep an eye out for.

A search tool like ack or grep are also incredibly useful to follow a trail of any interesting scent you might pick up.