If you're talking about pure efficiency, knocking out a ton of green and yellow quests has always been superior to doing red quests. The thing is, leveling is a lot different now than it was for the end of Wrath. tbh I don't even see red quests anymore in Cata, the way they have zone progression set up now. The only ones I can think of off the top of my head have been dungeon quests for multi-winged places like Stratholme or BRD. Maybe the group quests in Ghostlands, but those are completely trivial to solo even for clothies now in my experience.

Things are also much different in terms of leveling pets now than they were at the end of Wrath. Blueberries are beyond useless for leveling now, ever since they got nerfed into irrelevance because of things like people tanking Sartharion with them. They can't hold aggro worth a damn anymore, even if you have taunt macro'd and apply it manually, and even if your toon's not in heirlooms. If you are, sneezing will pull aggro. Which doesn't matter in the scheme of things unless you're fighting one of the rare remaining elite quest mobs because everything dies so fast. You're generally better off using a felpuppy or whatever for the extra dps. I only used my VW on my current lock until I got the succubus.