TL;DR is that I actually think it was a very balanced roster.
The only "real" questions, which I answer below, are 0 mages, 0 dps shaman, and possibly the resto druid (although as I explain below, I don't think that had to do with specific resto druid weaknesses, and more that SOMEONE had to be sat.)
I don't really find it relevant what specs of pure classes were or weren't there. Is it really a surprise, or worth discussing, that sub and BM weren't there?
The advantage pures (and DK's) have is that we have more than one DPS spec, so we can tune more specifically for a fight. So, yes, you will have fights like Anub where you won't see arcane mages in any early kills -- that says nothing about arcane, it says fire (which at the time was quite weak), was excellently-suited to the mechanics of the fight.
They used a feral and a prot to tank, which means you won't see a prot warrior (or a tanking DK) on a two-tank fight.
So basically it comes down to a discussion of how significant, if at all, any of these omissions are:
o Mages
o DPS Shaman (ele and enh)
o Resto Druids
IMO it doesn't say that much: you bring as few casters as you can because, like the previous two end-boss fights and much of the dungeons (despite GC's comments to the contrary), cleave/incidental AE is significant in terms of getting "free" DPS to get adds down. Casters have less cleave potential.
Warlocks are especially good at the fight, so you sit mages. Warlocks got sat from Anub at the start too. It happens.
I don't think there's anything fundamentally weak about mages here or that this is proof of them needing buffs or some major class weakness.
The DPS shaman is the part that I find most significant: both specs have been behind on DPS for much of this patch and they've never gotten the love, particularly the cleave/AE love, that other specs have.
Furthermore, enh shaman "oops" abilities are gone now that they have to use their one survivability cooldown for DPS, making them the only melee who can't save themselves: the others are far more survivable.
Ele falls prey to AE issues, plus the fact that you're stacking melee.
I believe the resto druid absence is actually a casualty of the shaman problems. You have 5 healers. You're going to take two paladins because you always take two paladins. So that leaves three spots.
Choose 3 of: disc priest, holy priest, resto shaman, resto druid.
Note that you have no lust or totems if you don't take the resto shaman. *Some* healer was going to get sat, and it wasn't going to be the resto shaman since neither of the DPS specs were attractive.
My guess is they valued the holy priest above the resto druid.