I really wish Blizzard would put some of their developers to work redoing aspects of vanilla WoW. In addition to the quest hole between 40-45, there's another hole from 50-53 which I'm just not nearing the end of on my Shaman. While there are some quests that fit into that small bracket--particularly in Ungoro crater, I detest that place with a passion. Nearly all of the quests there are collection quests, and the drop rates on it all seem to be vert low. For the longest time I would only do the first slate of quests available there, and now I can't stand to set foot in the place let alone run through the quests. My wife and I attempted to last night and promptly dropped everything and decided to bounce between Burning Steppes, Felwood, and Western Plaguelands. There's a considerable amount of travel involved, as you might imagine, but it's either that or spend hours upon hours in Ungoro. Sorry Blizzard, that's simply not my idea of fun.
In all honesty, while the quest hole from 40-45 is very wide and apparent, the hole from 50-53 is more self-imposed. Which of course by saying, would be seized upon by Blizzard in rebuttle of my contention. To that I say they'd have a point. But only to an extent. In all the quest brackets, with the exception of the 40-45 bracket, and the 50-53 bracket there is a wide assortment of quests. Even if you have to travel around a bit, you can generally run them as green because of it. You flow easily from one quest to another, and from one zone to the next.
Yet in these two particular brackets you seemingly have to travel much more than in other brackets, and don't generally run into green quests. Everything is yellow or even orange. The quests are your level or slightly above, meaning you will generally run through them a little slower than you otherwise would. And you will generally have more downtime in between fights.
In the 50-53 bracket It's Ungoro crater which I think causes the majority of the problems. If Blizzard would take a good long look at the zone, and redo some of the quests, and fix the drop rates on several of the quests the zone might be much more enjoyable and would go a long way in filling in that hole I see. Ideally I'd like Ungoro to fill the zone people generally go to from 50-52, with a couple quests added to Felwood between those levels to break up the crowds. And from 52 upward people could flow right into the lower level quests available in Western Plaguelands, Burning Steppes, or stay in Felwood.
The quests from 55-60 all seem to gel quite nicely--not that anyone stays in old Azeroth past 58 any more. So you really only worry about 55-58 at this point. And quite frankly, there are tons of quests in that bracket. So much so that you won't get to them all before you part ways with Azeroth and head through the portal.
Blizzard, if you're listening, Ungoro could use some of your attention. Sooner, rather than later would be best for all involved.