Pretty much, they have balanced the game around level 80 abilities, talents and gear.
Unfortunately, not everyone is now balanced at level 70.

Essentially, instance bosses (heroic/normal/raids) all now have 70% the health of what they had pre-patch.
So, even if your class/spec is a little less efficient now then you were pre-patch, your guild should be able to clear anything post-patch that it could pre-patch.

Of course some specs are insanely powerful at 70 now, and this makes it even easier.