Well, it's like this: Rogues in high end Ulduar gear have two real priorities: Keep Slice and Dice up, and keep Rupture up. Beyond that, they just work on fitting as many Eviscerates/Envenoms with mutilate rogues envenom refreshes slice anyway, which is nice) in as they can. Simple in theory, sometimes impractical in reality. In a lot of trash scenarios you had two real options:
1. Maintaining Rupture (preferably on multiple targets) and Slice and dice, blending FoK between refreshes rather than going for Eviscerates etc. In essence, keep the majority of your single target rotation and sprinkle in AoE whenever it wouldn't cripple your single target dps. FoK can apply poisons, so you're actually depending a lot on Deadly ticks to do your real aoe damage.
2. Face roll FoK. Before, you could often get away with this any time there were 3 or more targets and not really lose any damage potential. Your single target dps turned to dog shit, but glyphed and with the right talents and enough targets it worked, often well enough to top the meters.
But after the Fok nerf? It's usually much, much better to go with option 1 and rogues who never learned to approach it that way are feeling the pinch. Which, really, is a good thing for high end raiding and rewards skill rather nicely; I've barely felt the nerf in raids. But it's a pretty raw deal in Heroics, and I have a bit of a rough time keeping up with other classes there now because AoE is king vs. weak content.