I love doing this too!
Currently levelling a second character this way - doing every 1-10 zone before moving on to the 10-20s, etc, making sure I complete every quest available before moving on.

Sure, you end up doing a lot of grey quests this way, but I find it weirdly satisfying to always have every the absolute best piece of quest-reward gear available, to hit Ambassador by lv40, to be able to buy other racial mounts at lv29, etc.

I absolutely love levelling and questing. My main hit Loremaster shortly after Wrath was released, and this time I'm finding it especially rewarding to work on the achievement as I level. I'm hoping to have both a Horde and Alliance Loremaster before Cataclysm comes out so that I feel like I've really seen everything before the slate is wiped clean.

I'm leveling six alts in tandem. They're currently all on Searing Gorge (and level 53-54 when starting it, 55 at the end). They're all going for Loremaster, so I've been doing every zone in level order. It's not always the fastest, but after seven full times (my main is an 80 with Loremaster, explorer, The Exalted) through every zone in the game as fully as possible, I feel like I'll truly be able to have an appreciation of it when Cataclysm finally hits. The one thing it pretty much guarantees, too, which is important to me, is that I can solo almost all quests and content but the dungeons. The only thing that my main helped them out with was full sets of frostweave bags. I'm a compulsive auctionhouser, and trying to muddle through with at-level bags was not something I could tolerate. Aside from that, they're all financially independent from the main character, and they all have 2000-4500 gold apiece. I also don't use heirlooms. I suspect (though can't prove) that it would actually decrease my XP by pushing me to be "grey" in zones prematurely because of the extra XP I'd get from the items, and as such, I'd not be milking those zones for all the XP they're worth. At best, it would be a wash. I've got enough Champion Seals saved up to outfit all of them if necessary, but I haven't used them.

I hated the feeling of indecision on my original character (which I deleted after a break I took from the game back in 2008) about where to quest and what to do and what to kill and which instances to run. So I thought, why not just make a list, and do it in order. It's more fun to always feel like I have a goal to work on rather than just "level as fast as possible". There were people in my guild who thought (and still do think) I'm insane for even attempting this, because the quests are so boring. I fail to understand how running the same dungeons repeatedly or leveling in a battleground or always hitting certain zones because they're "best" for leveling isn't equally repetitious.

That's how I did Loremaster--all the zones in level order. It's useful because you don't miss any breadcrumb type quests that send you from one zone to another, all of which go away once you do the followup, and all of which count for the achievement.