I remember back in 3.1, when Ulduar PuGs took flight in a serisous manner. No one had ever bothered checking up extremely hard on those Sartharion or Naxxramas PuGs, you came in and did your stuff, it was usually just good. But back before ToC5 would load out everyone with their first taste of the truly "welfare" epics, people were still kitted out in 213 gear for the 219 content of Ulduar (10). And more and more often people would ask you to link your achievement "Epic" in addiotion to the relevant achievements for the instance in question, to ensure that you truly were kitted out for the task ahead.
(which really was something of an L2P issue - everyone and their dog knew that before Mjolnir, for most melee classes and builds, Darkmoon Card: Greatness and Mirror of Truth were the best in slot combination - both being ilvl200)
Then 3.2 arrived and everyone with the patience to save up 2 Triumph badges every day, and consistently run VoA, would soon get their hands on more or less full 232 armour, with Edge of Ruin, Marrowstrike or Aledar's Battlestar on top. Suddenly the "Epic" achievement wasn't really worth that much anymore - anyone with the will, patience and pure masochism required to go through heroic ToC5 every single day until they got what they needed, now had the hardest part of "Epic" covered. And a new kid started getting known around town - his name was GearScore.
To this day, I don't know how to calculate my GearScore. I know that when I check the regular websites who "audit" your gear, the average raider lie somewhere between 2600 and 3000, depending on hardcoreness. What I like about this "auditing" service, is that the commonly accepted best in slot gems and enchants count as the max score, with options and downgrades often listed as available, but with a lower score. It's not a flawless system, but it'll tell you if a player has put his Nightmare Tear in the best possible socket, and if he's using a +85 Attack Power instead of Berserking, or if his chest has +8 or +10 stats. It's a fast way to check if the player in question has average or above understanding of gear setup.
GearScores, however, tend to be somewhere between 5000 and 6000. I have no idea how they come up with this number - is it the sum of all your item levels? Or is it the more traditional audit concept with all values doubled to look very large and impressive? I honestly haven't got a clue. Maybe there's a site calculating it from your armory. Maybe there's an addon you use that tells you. I don't know, and to be truthful, I really don't care.
Because the last time I joined a PuG asking for GearScore, I talked my way around it (you'd be surprised how many times just linking DBB + 25 man "Salt & Pepper" is enough to let you in on just about anything). Said PuG had a Death Knight tank boasting a GearScore of over 6000, so I thought, this should be safe and fast. Until I stood there inside the instance, seeing the words "the Patient" over his head and him dualwielding that 1.70 tanking sword from Pit of Saron (a.k.a. the new Peacekeeper's Blade if you will). You don't even want to know what runes he'd engraved them with, but rest assured that his absolutely groundbreaking tanking build was a solid 0/71/0.
We wiped twice before RL left. I may or may not have joined him.
So what's the lesson here? That GearScore in no way can tell you if a player knows his class, the instance he's entering or even how to gear properly. Frost badges have been around for long enough that everyone making daily heroic and weekly raid a high priority, now can have 2-3 items of ilvl264. Add to it the available items of 245-level, and the Conquest T9-sets and IC5-weapons are soon the things holding your GearScore back. How can such a random number even be remotely close to an indication of a player's skill?
I still don't know how to calculate my GearScore. For all I know, it has a smart enchant- and gemming system that in a very clever manner calculates a precise and analytic number based on socket bonus utilization, programmed formulas indicating when a socket is worth it and when it's not for the talent tree the character's specced into. Maybe it even rates talent builds based on your gear setup? "No Resilience and still two points in Divine Purpose? Naughty naughty, that's ten negative points for you Sir!" But I kinda doubt it. 0/71/0, you know. And guess what?
He had +8 stats on his (crafted BoE ilvl245) chestpiece, too.