I think all of this is a little different than others. I raid with 2 guilds atm. One is a server first progression guild that has gotten all raiding server 1sts on our servers. My alt raids with a casual guild and has pugged raids and raided with other guilds as he hit 80.
The difference is quite noticable. It's not just skill but it's attitude and committment. When I was trying to get my alt gear pre-bk, and post 3.0 patch (because content was easy), I'd also raid with casual guilds.
Casuals have a different mentality altogether. Example, after killing a boss in BT, loot to 15mins to divy up, then they'd take a 10 min break after that.
Raiders typically have the mentality that once the boss is dead, it's time for trash, and do loot while we move. It's about not wasting time and keeping people involved.
Raiding is about everyone not only reading up on strats pre-fight, but listening on vent when a new tactic is tried. Example on Malygos p3, when you tell everyone to get in a tight group so the heals will hit everyone, and then when p3 starts 5 people die becvause they are in melee range, and another 5 are around the other side not gettting heals and wonder why they died. That's casual, especially when they do it 5 times in a row and still don't understand the concept.
Raiding/progression guilds, this will happen once, and it's fixed. If it's not, then there's a problem, and those toons don't get invites.
Raiding toons are also about personal responsibility. Sure content isn't the most difficult, but raiders understand (without being told) the concept of flasking, pots, elixirs. They understand the need to improve not only their gear, but positioning, rotations, etc, for optimal tanking, healing, and dps.
Casuals dont' really worry too much about dps and the need for a minimum 3.5k on Sarth3d and getting versperon down as fast as possible, as another example.