Just to put a disclaimer on this first. I came back to raiding in MoP from burning out in Ulduar. Did Normal ToT with one group then changed just before Soo to do Heroics and am currently at 13/14h. I think it's great that there a numerous levels of difficulty and that everyone can test themselves to their own skill level.
What I've found in doing PuG's for normal or flex is that a players ilvl does not meet their perceived skill level. Getting 553 tier from Celestials and 559 items from Ordos along with the 608 from the Legendary Cloak has inflated peoples ilvl far past their experience in LFR or flex. This leads to frustration for others who believe they should be better than they are and from the people themselves who find themselves being raged at or kicked from a group where they met the requirements.
World Bosses are going to be included in WoD so there is no easy way around this. One solution I can see is for the WB's to drop ilvl gear that is equivalent to the difficulty that the player has cleared. Not perfect I know so it would be great to hear from other people their views on the matter and any changes, if any, that they would make.