What kind of ads do AdWords don't like?

Hi, I read policy of adwords but still I hear time to time from people that their ads got declined even after following rules bla bla. So, wanted to ask your OWN experience on adwords on promoting certain niches and what things led some of your ads got declined?

Also, I believe using cloakers like JustCloakIt, Leadcloak can and will get you banned?
I mean cloakers can get you banned?

So, what kind of rules should I follow while trying to promote any niche?

Anyone on this?