To a degree, every expansion is a gear reset.

Burning Crusade level gear was such a huge upgrade over classic gear.
Aside from set bonuses, the level 61 greens blew away the level 60 purples.

Wrath of the Lich King, was a slight gear upgrade.
A lot of my toons were still in Tier 5/6 raid gear, at 75th for most of their slots.
Sure the end game gear was higher, but the initial quests did not blow away the 70 raid gear.

It looks like Cataclysm is a major gear reset, similar to the Classic > Burning Crusade jump.
The game is being balanced around massive health pools.
I'd imagine the mobs in level 85 dungeons will hit like a truck, at least compared to the level 80 dungeon mobs.
And with such a huge health pool, WotLK dungeons will be a joke.