It doesn't feel very good to be completing grey quests and artificially stopping your progression isn't good either. Traditionally you near the end of the zones quests when you are near the level of the next zone. All I suggest is that there could be an option. There is no more flow to the zones, which I imagine must annoy the developers that worked hard re-doing the world.
I'd be all for an NPC that gives you a -25% experience buff or something, but it really does beg the question of "what's the difference"?
You either choose to have to work your way through entire zones of quests because you need the XP to move on, or you do them because you want to despite the fact they're all green/grey. If for the later, you got a mod to remove quest coloring and turned off XP gains from your combat log, would you even notice the difference?
If Blizzard implemented the option, it would be de facto supported - so if people discovered that there's a "snag" somewhere in the leveling process, it would seem up to Blizzard to "fix it" - whereas the later option is clearly a personal choice.
Meh, I like the new lvling. Leveling low lvl chars is fun! But once I start getting to Outland/Northrend... jesus. And don't get me started on MoP... I have 2 90s because I find the 85-90 experience to be terrible. It's cool up to 60. There are times where I don't care that quests don't give me that much xp, I like the zone and I keep on going, because I think I subconsciously tell myself to actually enjoy the 1-60 content before I go all suicidal come Outland which starts with 10 consecutive Ramparts runs (still an upgrade from BRD, the only instance you randomly get 50-58?!)