The better the code is, the fewer comments it needs. In my work I've found again and again that the more comments there are the worse the code is. There are even code quality tools that use this as a metric for identifying problem areas in a codebase.
I do agree that it's not a two-way relationship: I've encountered many terrible codebases that have no comments. That suggests to me that the current Ohloh metric -- that more comments is good -- is the wrong way round. Many comments is a bad sign, but few comments doesn't provide any information.
As for test coverage, there are many test coverage tools that can determine how well tested the code is. Test code that itself is not covered by the test run has a negative effect on the maintainability of the code, so should count against the project.
Of course, test coverage itself only indicates where there are problems -- where code is not tested. It cannot indicate whether the tests are actually testing anything about the code they are exercising. There are mutation testing tools (e.g. the Jester family of tools) that determine whether the tests are actually doing anything useful, but they are computationally expensive.