I would blame the linearity more than the 5 levels.

Cataclysm is akin to a movie. Fantastic storytelling, but from the second time onwards it becomes too repetitive. You can't even skip a single squirrel fetching quest else the game won't let you progress.

Whereas in BC and WotLK you had a nucleus of essential quests each character needed to complete, with the other ones being truly optional, in cataclysm almost every single quest is mandatory. Whereas in the previous expansions completing all quests in a zone felt like an achievement, in Cataclysm it's a milestone.

The difficulty is another factor. Too much head banging leads to frustration, which leads to very quick burn out. It's what happened to me even before getting to raids; I only ran heroics in PUGs, and pugging heroics was simply not viable in the first month.

BTW, while it can't be completely trusted, if WarcraftRealms' data is correct, burnout levels after just 3 months of Cataclysm should be already on par with burnout after 18 months of WotLK. Not a good sign.