In Asylum of the Daleks, the daleks have assembled in a parliment, showing some form of governance other than totalitarian rule by the Dalek Emperor.
Why do they have a 'Parliament'? Is this a retcon, or a new development?
Well we don't know what year they're from. It seems they might have adopted a more democratic form of governance in a future timeline.
There are some indications that these Daleks are from a future timeline:
They are more advanced technologically (nanotechnology that allows them to convert dead or alive beings into Dalek drones.
They also seem to be more tolerant when it comes to racial purity (after all they did "allow" a human to be turned into a Dalek).
Hopefully we'll find out for sure in the next couple of episodes! :)
It's a continuity nod to the Daleks working with Churchill in Victory of the Daleks. As he famously said:
parliamentary democracy is by far the least efficient means of exterminating lesser races, excepting only all the other ways that have been tried from time to time.
Well nearly - original version of the quote is here . Presumably it got altered one of the times the Daleks were erased from history.
Several have mentioned a hive mind - this was an incorrect term used by Oswin, who then corrected herself. It might be easier for you to think of Pathweb as the internet of the Daleks. It therefore doesn't negate the need for a command structure.
No specific reason is ever given for the change from hierarchy to parliament in a canon source. One logical reason, though, can be seen in the casings. They have combined both new Paradigm Daleks and what appear to be War Daleks. The most likely source is from the Daleks trapped in the Void in "Doomsday", who were pure as well and would have their own command structure. Rather than a fight for dominance, they may have created a parliament to combine their forces.