I think the biggest problem golfers face is that they have developed such poor, erroneous or non-consequential concepts that they believe to be quality, correct and important concepts of the golf swing that they never improve. One goof over at the GolfWRX.com forums would lead the world to believe that all you really need to do is practice the fundamentals and all the fundamentals are is a proper grip and a proper address position and you're all good to go. Furthermore, there's really only one way to properly grip the club and address the ball in this goof's opinion. This is really hazardous teaching because it ignores a ton of other fundamentals to the golf swing and if somebody like Lee Trevino or Moe Norman were to ask for some advice, this good would be changing address positions and grips that don't need to be changed.
One of the worst swing theories that I hear all of the time, something that drives me absolutely batty is when a high handicapper starts talking about 'keeping the left arm straight at the top of the swing.' This is a complete fallacy with little importance, unless you are really bending the left arm. My guess is that this started with people way back in the day trying to emulate Ben Hogan's swing. But, here's a bunch of darn good golfers and let's take a looksee at their left arms at the top of the swing.