Started playing WoW just as BC launched. Really enjoyed it throughout BC. I was lucky in that I met some great people where we spent most of our time on Vent just chatting and having fun in-game. We made a raiding guild which was successful and this was probably when I was at my peak enjoyment when playing WoW.
When WOTLK hit a lot of my friends quit due to IRL stuff such as starting university, but I was still at school so I still had the time to put into it. It was never the same though, for two reasons:
1. Friends quitting
2. The game (for me) started to go downhill as it grew in popularity. I'm by no means a hardcore gamer but they took a lot of achievement out of everything. The levelling process was incredibly easy and max level could be reached within a week. This created the idea that the game was ONLY about the end game, and the levelling process was just a means of getting to that end-game without enjoying it in the process. They added quest pointers to literally tell you where to go to do quests which took out any feeling of exploration or wonder from the world.
LFR was the final nail in the coffin for me. Any immersion I once got from the fairly difficult bosses you encountered and satisfaction I got from killing said bosses was gone. I remember seeing Kael'thas in Tempest Keep for the first time and fearing the fight ahead. Now you can do current raids in LFR and face roll all the bosses with little difficulty with a huge margin for error. Even the hard versions of the raids lost their appeal after I had already seen the bosses in LFR.
Tl;dr: Used to play loads with mates then they quit. Game lost any sense of achievement or immersion it once had and I lost interest.
Edit: Grammar