Everything I know about constructive criticism came from one of my college professors. The man had an ego the size of Nevada but his explanation of what criticism is and is not was one of the best lessons I've ever had in my life.

Re: grammar, I have to admit, I pay a lot more attention to whether something sounds awkward than to grammar per se

I'm entirely the same way - which makes explaining to the author why I think something sounds wrong awkward. ("It just does, you need to trust me" only flies with the people who trust me implicitly.)

Is Language Log something available to read online? It sounds interesting.