There are a couple of different dimensions to its popularity.
First, it caters to the "quick and easily understood" information need. Everyone is reduced to a number that describes their worth. We can all very quickly tell that someone with a score of 5001 is better than someone with a score of 5000. This is very attractive to the "drive through" generation that makes up a very large percentage of the player base. Another way to put it, perhaps more troll-ish, is that GearScore caters to the lazy people who don't want to look at specs and understand why a 232 or 200 trinket is better than a 251 trinket.
Second, the addon uses some measurable facts to determine the GearScore number, namely the item level of the gear being worn. This makes it legitimate in some sense, and folks can attach onto this legitimacy easily. After all, someone wearing all 232 gear is going to do better than someone wearing all 187 gear.
Finally, the addon lets players think they're better than other players. It says "Hey you with 5000 GS, guess what? You're awesome compared to that scrub with 4500." People like feeling better about themselves, and in a game where gear no longer determines a person's worth, GearScore is the ultimate kick-back to the old days when someone in full tier-3 could walk around like a god.
And remember what they say, "If someone asks if you're a god, you say, yes!"
GearScore is saying yes.
"Why MUST we go to Black Temple to kill Illidan? From the quests I have done, I got the impression that the evil/demonic things Illidan had done was in the past, unlike Arthas who was zealously dreaming about dominating the whole world. There's no indication (so far as I know) saying Illidan was planning a new invasion to Azeroth or something like that, although the demons at the dark portal could have links to him. On the contrary, Illidan was upset at the fact that Azeroth people simply wouldn't leave him alone. So, why did we have to go there to pick on him?"
You don't just let bad guys live out their days and expect them to do nothing, especially immortal pricks like Illidan. And while he wasn't going to go all spaztastic on Azeroth right away, he did want to extract his revenge. Plus, you know, he was sitting in the very lands that the Draenei and Orcs once called home. That's pretty good reason to want to kick his butt to the curb.
"Why are the statistics in the tab statistics in the achivements everything but acurate?"
They are mostly accurate. There are errors if your character was created before patch 3.0.2 when the statistics started being tracked. For example, the stats on my warrior tell me I've never killed Ragnaros, which is off by about 100. The other thing is that not all damage-in/damage-out/actions count for everything we think they should -- what Blizzard thinks is another matter.
"While leveling alts in Hellfire, it dawned on me I did not know where the Fel Reaver comes from. Who made the Fel Reaver, and what is its purpose?"
All we know is that the Fel Reavers were made by Burning Legion engineers to destroy Outland. Those little oompaloompa like dudes that you kill in Shadowmoon Valley? They're the ones building them.
Have questions about the World of Warcraft? The WoW.com crew is here with The Queue, our daily Q&A column! Leave your questions in the comments and we'll do our best to answer 'em!