In TBC, it was attunements coupled with reputation: Blizz decided dungeons was not easily accessible to the core of their customer base, so they fixed that in WOTLK.
In WOTLK, it was the quick and easy dungeons that everybody could do very quickly and got bored of just as quickly, so they fixed that in Cata.
In Cata, it was the going back to difficult and longer lasting dungeons and disjointed zones, so they fixed that in MoP by going back to the WOTLK type dungeons and a single, large continent.
In MoP, it is the release of major content via dailies. Dailies is the thing they are going to "fix" in the next expansion. You heard it here first.