Many months ago (I forget which patch exactly), Blizzard put in new quest hubs in Dustwallow Marsh that really filled in a huge hole in the level 30 bracket. Up until that point you were more or less forced to head to Stranglethorn Vale as soon as you hit 30. And there you probably remained until you were 40-43. Questing there was monotonous, tedious, and required vast quantities of travel. Not only were there (and remain so) quests that require you to travel back and forth between the very northern parts and the souther n parts of the zone, but also require you to travel to other zones. The zone is so large that having to walk from Nessingwary's camp to Booty Bay could easily take 15-25 minutes. If you were killed by the progressively higher level creatures lurking along the road side on your trek south, then that only added to the misery.
STV is still there, but the recent change to earning mounts at level 30 now has greatly improved questing there. But it pales in comparison to what you experience in Dustwallow Marsh. The quests in Dustwallow Marsh not only give you more XP, but also give you vastly better rewards. But most of the quests there start at the mid-30ish bracket, so before heading there I usually head into STV at 30 and do the quests at Nessingwary's, the rebel camp, and a few odds and ends in Hillsbrad/South Shore before heading into Dustwallow.
I end up having to travel a little more doing it this way, but the experience is more pleasurable. However that feeling largely ends when I've completed the Dustwallow quest chains and find myself around level 40. The question then becomes "where I go from here?" Unfortunately there simply isn't a very good answer to that. Ideally you would want to keep your quests in the green, or perhaps yellow. But there are fewer zones to choose from (Tanaris, Feralas, Azshara, Hinterlands, Searing Gorge, Blasted Lands), most of which start their quest lines in the mid-40's. Quite probably 3-4 levels above you when you step out of Dustwallow Marsh. If there is one thing you do not want to do at that point in your levelling, is to have your XP per hour plummet because the creatures are significantly higher level than you.
You probably end up doing what I do at this point, which is to bounce around, picking up whatever quests are closest to my level and running through them. Then moving onto the next zone. Having to do this adds considerably to the time it takes to level. Fortunately though, you end up having to do it for a hand full of levels, because by the time you are 44-45 you can find ample quests in the several zones that you can choose from. And the hole is largely gone by the time you hit the 50 bracket where there are a handful of zones that offer a great many quests to choose from.
When Blizzard introduced the Dustwallow Marsh patch they also reduced the XP required to level from 30-60, which helps a good deal. But I think and additional look in the 40 bracket should be made. Ideally I'd like to see re-worked quest chains in Tanaris, Feralas, and the Hinterlands to help smooth out the transition from the low 40's to about 46. No one--including an old alt-a-holic like myself, is remotely interested in remaining in Azeroth any longer than is necessary.
Blizzard has made it very clear that they intend to look forward and not backward. So Blizzard, help us move a little faster though the old content so we can look forward as well.