In our design, the pure dps classes (hunter, mage, warlock and rogue) should do slightly higher dps than hybrid damage-dealers all things being equal. All things are rarely equal. Player skill, gear, raid comp, latency, random luck and most importantly the specifics of the encounter will often favor one class, spec or player over another.This raises the question of what mechanics tend to influence dps. There are two basic types:
Direct. These are buffs like Storm Power on Hodir or Shadow Crash on General Vezax. They have an immediate effect and will obviously favor certain dpsers over others (high-crit classes for Hodir, casters for General). Direct mechanics are generally a positive increase in dps, although there are exceptions.
Indirect. These are primarily mechanics that force movement (although something like XT's Heart that favor a 30-second window of dps count too). These are present in practically every fight, and they generally decrease dps.
- Ulduar: Nearly every fight has something that forces movement, with the possible exception of Flame Leviathan and XT (if you get lucky and never get a Light Bomb or Gravity Well).
- Northrend Beasts: Snobolds and Fire on the Ground for Gormok. Poison and Burning Bile for the Worms. Moving to dps Icehowl when he's stunned.
- Lord Jaraxxus: Legion Flame, and movement to keep Nether Portals or Infernal Volcanos in range.
- Faction Champions: Yes.
- Twin Val'kyrs: Light and Dark Orbs, and switching Essences for a vortex/shield.
The problem with this is that fights with movement tend to favor pure dps, and most fights are movement fights. Not for player skill reasons, but because all of the pure ranged dps classes have an instant/high movement ability:
Warlocks: Demonic Teleport
These abilities aren't perfect or flawless but they are much better then what moonkins, elemental shamans, and shadow priests have - which is nothing. On any fight with a decent movement requirement (which is practically every fight) they give the pure dpsers an even bigger advantage then they might otherwise have. I never expect to beat a good mage on Hodir, because I know that mage can blink to pick up Storm Power, to avoid crashes, to get fire - the list goes on. I know that a warlock on Twins should beat me because he can teleport back and forth between essences as necessary, and he has an easier time avoiding Orb Surges. A hunter on Mimiron should beat me because he can jump over fire, and I can't. And the same holds true for most current fights - there are always mechanics that favor instant movement abilities, and therefore favor pure dpsers.
To a large extent I don't have a problem with this. The issue is that hybrids come under a double tax, once for being hybrids and then a second time for dpsing a fight with movement. If I can be within ~5% of a mage on a stand-and-nuke fight then I'm ok with that - the problem is when I drop to 10% or 20% below a mage because he has Blink, and I don't.
I hope that Blizzard fixes this in Cataclysm by giving all the hybrid casters some way to move around instantly, but I'm not super confident - it seems to be an issue that floats under the radar most of the time.