I have deep respect for MirandaEscobedo's disapproval of the first two expressions - kind regards is so frequently put at the bottom of letters and emails that I can't see much against that; it's a formula of politeness.
However, in organisations people in charge of a particular operation can be at once
1. Conscious of the importance of people's remembering things.
2. Conscious that people may have been told some time earlier.
3. Conscious that some people are forgetful.
4. Conscious that some people take offence at being reminded.
In response to this problem they write 'polite reminder' in the hope that they will remind the people who need reminding, while not offending the people in category 4.
In doing so they irritate people like Miranda and me, who dislike expressions which try to proclaim their own friendliness or politeness, rather than establish it by more conventional means. I think KaleNovice has to choose where she stands on this issue. I wouldn't be worried by a note which started Reminder, but it seems that some people would.