I know we're all supposed to shit on John Wayne for his abhorrent politics but I'm going to defend him on a few points. Everyone loves to proclaim that he was a coward during WWII because he didn't serve but 1. He was deemed too old and had prior injuries that had previously ended his athletic career 2. Republic Pictures threatened to sue him if he left his contract with them (read up on it) and 3. There is documentation that he enlisted in the OSS but the letter of acceptance went to his estranged wife's house and she never told him about it. Whether or not you find these excuses compelling, I would still hold off on exclamations of cowardice.
And while I find High Noon to be true on an emotional level, Wayne is correct that it is not anything close to the reality of the Old West. There are numerous instances of outlaws riding into small towns to rob and terrorize the populace and without fail the town always banded together and shot them to pieces. For examples, read up on what happened to Jesse James and his gang in Northfield or what happened to the Daltons in Coffeyville. Considering the time High Noon takes place in, most of the men in town would probably have been veterans of the Civil War and not ones to run from a fight. It's not a slight against High Noon because I don't think authenticity was the goal.
Also, Parke's info regarding sheriffs and marshals is a little off. There were US Marshals but most Westerns aren't about them. Sheriffs actually had a higher standing than the marshals we picture in Westerns. Sheriffs were the law for entire counties, while marshals were the law for towns only.