What is Well-Commented Code?
Far too late to join this conversation, but let me add my $0.02 anyway.
What I miss most in most source code are comments that communicate the architecture and flow of control at the module level. When you get the source code for a project, it's generally a flat list of files. Where do you start? main() is not always the right answer - what's needed is an overall look at the code base and a guide to how it was put together.
Comments in the source rarely address these questions, being more locally oriented as a rule.