I'm one of those people who played WoW for a good few years, then suddenly quit and I haven't wanted to play it for more than a year.
Option 3 would make the most sense, especially from personal experience. Before level 80, there is always a clearly defined objective (leveling to 80) with multiple pathways to accomplishing said objective, and this is the really fun part of the game. Once you hit 80, what more is there? You're suddenly just another face in the crowd again.
One of the things I just missed most when I was high level was the lack of recognition for challenge. When I was a level 30 Dwarf Warrior and I killed a 36 Horde character that had the first strike advantage... that was accomplishment. That was a demonstration of pure skill. My guilds generally gave out promotions for 2 things, devotion to the guild and skill in guild activities, so when my Horde Enhancement Shammy healed through a very difficult dungeon for healing specced characters with little problem, I was up for promotion. That was accomplishment. There was no accomplishment at level 80. Your guild killed some boss? Good for you. Made 2,000 gold? One of my alts has that much.
I think you just feared that lack of personal accomplishment as much as the drastic change in playstyle. Essentially, you feared hitting the end of this really long journey. I myself never made it to level 70 (I quit before WotLK came out), and I'm perfectly happy that I never hit the end.