Staying a little less each time?
Probably this.
I came back for Cataclysm but once I got my Hunter to Level 85 and tinkered around with a Goblin Rogue for 20 levels...I was done. It's the
exact same end game that we were doing back in Burning Crusade.
There's also what I call the "Gear Deluge". Back in Vanilla, T1-3 were progressive Tiers of difficulty. Meaning you had to acquire one before attempting the next. It made Gear valuable and that status didn't change as time went on. Naxxramas (60) was only seen by 2% of the game's total population while the majority were still banging their heads on Onyxia. I remember when PUGs of LBRS and UBRS were laughed out of Trade chats.
Now? Wait a few months, and Blizzard will effectively hand the population the Gear level they need to tackle the newest dungeons. I understand the reason is to ensure the most people see the content but the unintentional consequence is Gear is Worthless. A full set of Tier Gear will get you what? Two? Three months of play time before it's time to trade it up for the next best thing?
I remember when seeing someone walking around in full Tier 2 was a jaw dropping experience - not just for the person in the gear, but the newbie too. People approached to speak with you. It was like seeing a celebrity. Now? Everyone's running around in the same gear, just colored differently.
I'm not really arguing for a caste system, but there's no sense of accomplishment any more.