It's not necessarily the gearing up that made it harder. It's the massive HP boosts just from leveling. Getting more gear gives more dps, which still makes it easier, even with the increased max hp. Just gotta get healers that know not to try to keep people up to full...
I recently did anub25 hm for fun the other day, and it's not that bad to be honest.
we did have some well geared people in the raid but they were kinda offset by people in full blues. even so, we brought anub to phase 2 in ~1 minute.
from then on, it was just some light AOE healing (to keep people hovering at 10-20% hp) while EVERYONE blew all cooldowns, including heroism/lust.
however, if you don't manage the adds properly in the first phase, you're going to have problems with them killing the raid.