Originally Posted by GSH

It depends on how you look at attunement. Attunement is a symbol that "I am ready for this instance." It's a very common question for people or guilds who are new to raiding: "Are we ready for Karazhan?"

A simple answer is "You are ready if everyone is attuned."

However, for further raid instances, it's much easier to state readiness in terms of the previous instance:

You are ready for BWL when you've beaten MC.
You are ready for AQ40 when you've beaten BWL.
You are ready for SSC/TK when you've beaten Gruul/Mag.
You are ready for Hyjal/BT when you've beaten SSC/TK.
You are ready for Sunwell when you've beaten Hyjal/BT.

TBC-style attunements make this explicit, but they really aren't necessary. However, for the first raid instance, an attunement is necessary because it's a lot more uncertain as to when you are ready, *especially* for a guild or player who is new to raiding.