And you've given entirely different level to ignorance. On several places in my response I stated that I know my score is not that high, and I know the numbers are not a completely valid representation of players skill.
So once again I will reiterate the most important points
1) Yes the community is ridiculous with their requirements more often than not (but since you are complaining I suppose you would have no problem taking a random 880 guy into NH heroic)
2) People need to pick their group based on something for which M+ score is quite a good metric. Requesting 2K+ rating into +15 is a bit ridiculous, yes -- however something 1800 rating currently means you've done all the dungeon for +15... cathedral and kz are harder.. so say 1700 is a reasonable requirement (comparable to requiring curve in pug for hc Gul'Dan).
3) Your raid accomplishments may give you a better "visibility" but by no means do entitle you to be picked. If I am presented with 2K m+ score guy and a guy with 700 m+ score who killed 10/10M I will pick the first one. Not because he is a better player, I will do it because evidence indicates he is more familiar with the content and thus likely to perform better. In the same manner if your guild looks for a new member of your raid group they will likely be more interested with guy who got a decent raiding experience and kills compared to guy who runs higher mythic keys.
Another fun fact: In a spirit of "everyone here on mm-champion is 10/10M a has top 0.1% of pvp rating" it seems everyone here is running 20+ key on time since that is where the "hard part begins". They keys are not that easy starting from +17 I would say -- especially if you don't have a stable totally coordinated group. Personally I would consider keys up to +19 fairly easy (again depends on the dungeon and affixes), however that doesn't mean that +17 tyranical upper karazhan is easy to beat. If I am the elitist here I wander how all the "You are shit unless you are the best" guys should be called.
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Thank you! And yes, you are totally correct -- ideally I should check the specific dungeon. This is yet again a matter of efficiency -- it's easier to check a number than to go and look at specific dungeons. It makes perfect sense, however I think that expecting an average Joe to do this amount of "research" about people is unreasonable -- especially if he does have unreasonable requirements for lower level keys, he likely wants to get carried.