Think of it as a way of measuring player progression beyond the current level cap. Instead of an arbitrary gear score, it is a score directly based on your current level in order to have some context. A level 80 player in all quest greens and blues would expect to have a score between 81 and 82, while a player in all Tier 10 and equivalent gear will have a score over 86 or 87.
Scores are determined by calculating an individual score per item, based solely on its item level (ilevel), and then adding the sum of those scores to your current level. Roughly speaking, the formula for determining the score per item is:
(ilevel ^ 2 / 213) / 1000
213 was selected as the baseline for determining each individual item score since it is the ilevel of Tier 7 loot. This is intended to help distinguish those who are in Tier 8 and higher content, while simultaneously lowering the score of very old gear.
I do recognize that many best-of-slot or almost-best-of-slot items have low ilevels, including some blue and badge reward gear. However, considering that it would add a great deal of time to build a table of comparable items for each class and spec, I figured that can wait for a future version. As of now, there is no distinction made for best-of-slot items.
No distinction is made between PvE and PvP gear. Considering the stricter requirements Blizzard has placed on the higher level PvP/Arena gear, I see no reason to not treat them equally.
No, it does not. Shirts and tabards do not increase or decrease your score.
I'm about a year late, but I've finally updated this site for Wrath of the Lich King content. Since scores were accumulating with Tier 7 and higher gear based on the old formula, I decided to normalize all scores for level 80 players.
That's not a question, but no, this isn't really intended to be very sophisticated. It's just a fun way to look at your cumulative gear and translate it into an approachable number that you can compare with friend and foe alike.
Your Feedback and Comments are Welcome! This is a work in progress. daniel@spectrum5media.com