[Qaedain]

I think it was confluence of many factors:

• WoW's introduction attracted all the casual players that were simultaneously necessary for EverQuest's survival, but turned off by the relative difficulty.
  
• Sony's money engine hit full steam, which abandoned the very heavy MUD/D&D influence that made Vanilla+RoK+SoV feel so coherent and complete.
  
• It was the first large MMORPG. Indeed, one of the first titles even to be considered an MMORPG. By the beginning of its decline in 2004 and 2005, it was likely that EQ's number was simply "up." People wanted something new.
  
• Successive post-SoV expansions moved the focus further and further from the solo and group content that made the game feel so "personal." The game became focused on the guild and their microcosms, which killed the need for a server-wide community.
  
• Starting with Planes of Power (though I love that expansion), and continuing with Gates of Discord and Omens of War, the game became quite hostile to players. Zones were densely-packed with mobs that could kill virtually anyone in a quad, the number of see invis mobs increased dramatically and zones felt relatively smaller by virtue of their density. Intra-zone travel became challenging.
  
• The preeminence of keying and flagging. There's a time and a place for keys, but they should feel like a very special accomplishment. It should be meaningful for the player to receive a key, and that's done by incentivizing through the quest to obtain it and the rewards that lie beyond. Unfortunately, Sony started putting flags and keys on everything, which forced players into the rat race of repetitive timesinks. Tipt? Vxed? VT? Zones should have natural barriers to entry--requiring a Wizard, difficult mobs, requiring a good group comp--not artificial ones merely to prop up content that would otherwise be dominated in short order.
  
• The death of high fantasy seemed to cement the decay that began in 2004 and 2005. After Velious, the game diverged into the SoL/GoD/OoW content that didn't look anything like the high fantasy D&D/MUD players are accustomed to--the same theme that made the original EverQuest so flavorful. This feeling made a resurgence in Planes of Power (I liked this expansion), but it was hamstrung by keying and hostility to the player.
  
• Server transfers and renames late in Live also had a deleterious effect on the game. A move to keep up with the Joneses (World of Warcraft), EverQuest and WoW both gave people the ability to shed their bad reputation, destroy the community or find greener pastures at the drop of a dime. The magnificent thing about EverQuest is that you were your name: your actions, habits and words followed you everywhere. You could not, after all, change your name or hop servers after you ninja looted a Trak BP; you repented mercilessly, or you got blackballed. Keeping a consistent user base is essential, and EverQuest succumbed to the dollar and forfeited that.

I could go on and on.