That's true Schmedz, although it falls down when describing new skills they have acquired. I might say "X can now solve simultaneous equations" as a good thing because it's something they couldn't do before. But I agree that before a behaviour it's probably not great.
We had 'mischievous' on DD's report last year. That's worse than lively! In fact it is not even properly disguised teacher speak for naughty! They did also say she was delightful and full of fun too (presumably when she isn't being a royal PIA)
DS only ever gets 'quiet' and 'a pleasure to teach' which means he is so silent he is all but invisible and this is a welcome attribute as far as the stressed out teacher is concerned. He has had this on every report for 9 years now!
Determined is good though - perseverance is a good trait.
I have always loved searching my children's reports for this year's way of saying "mini seeker is doing very well, but would do much better if he/she would shut up and listen for the occasional 5 minutes"!