The problem is raiding successfully. Guilds are social creatures. Imagine that a guild is your family and raiding is your job. Do you see your co-workers as your family? Are they the ones you want to talk to socially and be in constant touch with? Maybe. But, would you work with your family members? Would you want your mom to do what you do, be your boss, or be your employee? Unlikely.
I have been enamored by the Leftovers concept. It detaches the progression that raiding demands from the social stigma of guild hopping. The fact is that raiding does not demand lots of in-game socializing and contact. There are very few practical advantages to building a raiding group around a guild. There are very good reasons to build your social life around a guild. Before I saw this Leftovers concept, I had an idea for the No Guild Guild. Individual members in WoW would join a guild they liked, or be guildless. But, they could join an offline community whose sole purpose was to coordinate raiding and progression in the game. No more social tangles of having to leave your guild to raid. No more guild shopping to find a guild that offers both social interaction AND raiding at the right level for you.
Detaching guilds from raiding is the future.