What is the difference?

Several people on here recently have been accused of being 'curtain twitchers' 'peg police' and other things because they have commented on what they have seen on site in terms of pitching,behaviour etc.   I do enjoy people watching wherever I go - including on site - and I do notice things as I walk round but unless I walked round with my eyes shut it would be hard not to.

So, what is 'being observant' if you like and what is considered 'curtain twitching'?