As I mentioned in the GAF thread, I used that free week promo in September. Only one of my old friends was online, and he's the diehard type; he left our server awhile ago for greener pastures.
Most people get tired of the grind and general process of raiding. Fighting bosses, rolling over loot, seeing someone get owned by trash mobs...all that stuff is fun. But after years of mind numbing down time, or dealing with people who weren't prepared to raid seriously, etc...it takes a toll. And once you burn out there just isn't much for you to do, end game wise. Daily quests get tiring after only so long. I remember grinding Quel'Danas dailies nonstop, then grinding even more dailies in WOTLK (the ToTC dailies were super fun, especially the jousting). After WOTLK I was basically done with that concept, although Cata shook things up a bit by offering more variety/randomization to daily quests.
I had thought the speed runs/challenges would endear me to MoP, but they didn't feel meaningful in a random group. We used to time ourselves during daily instance runs back in TBC and WOTLK, stacking groups with the best players to quickly beat the new daily at 2AM and then going to bed, preferably after laughing at friends stuck in slower/worse groups. Maybe a way to fix that would be to offer more badges/rewards for challenges completed without using the LFG function. When I last logged on my server the general and raid chat was dominated by people looking to pair with 2-3 people for a LFR run, which just kills my soul to see. Bleh.
What does GW2 do for endgame, outside of dungeons?