I have to admit... the examples of bad job postings you mentioned seem rather simple.
To answer your question specifically about what types of job postings I flag, and which I feel oDesk responsively removes...
I have flagged a lot of "job postings" that were actually postings from contractors looking for work. ("Job postings" with their resumes and CVs such.)
I have flagged a lot of job postings which were not written in English, or which were duplicate job postings, or which were clearly "spam" postings.
I flagged a job once that had a link in it that downloaded and had a virus in it. The job had 100's of applicants. I also sent a note to customer service. It took them 3 weeks to remove the job.
I think that should have been taken down immediately but it wasn't.
@Suzanne "I flagged a job once that had a link in it that downloaded and had a virus in it"
Been there, done that. Whenever I see a job that has a direct executable link in it, I run away like it's the plague.