Actually I was not really saying why I didn't like it. I just gave an acceptable excuse which happened to be true.

Well it would have worked for me :-)

Would your impression have changed if I said that in 2 weeks I was going to be transferred to be working full-time under a manager at a client who I had grounds to file a sexual harrasment lawsuit against?

Nope. That would have worked for me too. It would send some HR departments running scared I'm sure, but would you want to work somewhere where they run scared...

But yes, I agree that "I hate my job" is a legitimate red flag. However I don't make nearly as big a black mark out of it as most people seem to (as long as there is a reason to hate the job).

It's just the phrase "I hate my job" or variants of it. It's been my experience that people who say that first tend to be asshats. This seems to be other peoples experience too.

Interviewers are only human, so it seems better to avoid phrases that might raise asshat flags. Give the reasons why you hated the job instead.