I don't think I've received a "we promise this is anonymous" survey from a hotel, but if I did, I wouldn't treat it as such.

Really the best advice if you do plan to submit such a survey is to take a couple days to cool down about the situation, then sandwich the complaints between a couple of slices of positive things to say about your stay.

Basically treat it the same way you would if you were complaining in person, which is hopefully polite but serious.