Things that I think are over-emphasized about headphones, or straight up misunderstood and/or utterly false, here:
03. "Forwardness" of a particular range of frequencies
04. "Recession" of a particular range of frequencies
05. Specifications of a headphone or piece of equipment being used instead of actual experience (this includes graphs)
06. Requirement, scaling effect and general "better with" of an amplifier for headphones
07. Impedance is somehow associated with higher quality or "audiophilia"
08. Reposting of ancient information that is not accurate about a headphone or line of headphones (Denon's screw, etc)
09. Too much guess work from reading, with fewer and fewer posts having experience behind the commentary
10. Burn in
11. Overhyped headphones that get re-posted as the singular greatness in audio with no experience with other headphones
12. Flavor of the month anything
13. Brand loyalty
14. Cable upgrades with differing metals in the context of low power and short distance (ie, headphones)
15. What hi-fi is, and/or what reproduction is, all things involving "what the artist intended"
16. Headphones that are only good for one or two things, yet still recommended & funded (ie, "good for gaming" "good for movies")
17. Compressed audio versus lossless audio digital formats & physical media formats (compare this to audiophiles with speaker setups)
18. $200 headphones being referred to as high end
19. Fashion headphone posts, both praise & hate (obvious bias is only interesting to read if meant in a context of humor)
20. Cost reflecting quality only
I find the biggest factor to be psychology and it's discounted or not considered at all when talking about audio in general.
The psychology of preference and it's involvement with what something looks like, something that is popular compared to less commonly known, associations of sound better and quality with something that is more expensive or commonly known, such as branding, or simply being posted more often and therefore somehow better because of that. In other words, if it's more expensive, is something you've heard of, and is the newest thing around or a well known classic, it will sound better than something that people state over and over sound inferior (basic herd mentality; note, this is not a negative comment or description, this is simply an aspect of psychology that heavily applies here and is often utterly not considered). There are a lot of headphones that are technically inferior to others yet sound better to more people. There's simply way too much, also, of an over-riding idea that if you spend more money, get a nicer cable, nicer amplifier, better DAC, your entry level headphones will sound better with your compressed MP3/iTunes collection. Of course if you want it to seem/sound better, it will to your brain. Psychology plays a massive, massive role here.