Ok, I was using MMOwned like crazy last week trying to nail 58 quickly on my lock. Since my account was banned with all my 60s, I had to start from scratch. I didn't really have any trouble until I hit 50, at which point leveling became the equivalent of watching grass grow. If you only have 1 or 2 characters and enjoy leveling, disregard this topic. If you have 5 or 6 and the thought of grinding Western Plaguelands again makes you sick in your mouth, then consider these quests for quick jolts of experience when you need them most.
I'll start around level 50, the only real quests worth turning in are the "kill this" quests in Burning Steppes and maybe a run or two through Maraudon or ST (maybe around 51 or 52). Honestly, I did the western plaguelands grind because I'd been on a long WoW hiatus. Any additive quests here are welcome. [you]
When you hit level 53 or 54 you open up some nice options. Of course there is the Un'goro grind which is wonderful, go to Marshal's Refuge and pick up all of the quests at once. They come together quite nicely here. Be sure to avoid Shizzle's Glider (or whatever), the Dino Scales quest, and maybe the Pterrordax quest (though I took this one) because those three have incredible time consumption to them. You get nowhere fast on those. Otherwise, tear Un'goro apart. Also head to Felwood, pick up the quests in the camp, along with the quest involving the Deadwood Furbolgs further north. These quests are easy also and reward huge experience.
When you're 52 I also believe the quest for the tablets in Azshara opens up. The first quest in the chain is "Stealing Knowledge." This chain is crazy, it will get you close to 40k experience (maybe more) and that is a huge load off your back when you've been grinding like mad.
At 55 I think (might be 56) the option to turn in Bone Fragments, Savage Flora, Dark Iron Scraps, Core of Elements, and Crypt Fiend Parts opens up in Light's Hope Chapel. Do this the first shot you get to, when it is still an orange quest. Go to the auction house, and shell out 20g (total usually) for 30 of each of these required items. Return, turn them all in for 35k instant experience. 20g for 35k experience? I'll take two!
Around 55 or 56 you need to be hitting Winterspring like crazy. If you aren't a kiting class, bring a friend, since the "kill this elite version of the other mobs" quests are so easy to find and complete it's mindblowing. 30k worth of experience if you do it at a low enough level. Also, don't underestimate Donova Snowden. If you kill the Furbolgs to the east of Everlook you'll eventually get an item that leads to a long but easy quest chain with her. Bring a friend again if you aren't a lock or hunter. There are delivery quests to be found in the Inn at Everlook, and these are an easy 10k experience that open up Eastern Plaguelands quests that, you guessed it, are easy experience also. Use Allakhazam for the finer points of these quests, but trust me when I say they are quick experience if you follow them diligently. Don't forget to grab the "Are we there yeti?" quest chain before starting these quests...it completes itself if you kill yetis as you go, and is more quick experience. Very easy to get 56 doing these quests.
Finally, I'll lead you to 57. Essentially, this is where I'm at on my lock, and I plan to hit 58 by Monday. Sadly, I fear I may have exhausted my quest options, but for now let's worry about 57. One word...Silithus. It may seem tough, but god is it easy. Pick up all the quests in Cenarion Hold you can, excluding killing deathclasp unless you bring a friend (friends are bad, they steal experience from kills you get along the way). Grab "Securing the Supply Lines" and "The Twilight Mystery" plus "Deadly Desert Venom." Also, for Horde, head to the new PVP encampment and pick up a quest for Silithyst transportation. Grab a silithyst (there's almost always one nearby) and run it back to the camp for a quick 6k experience. Follow these three quest chains for a huge dent in your experience to level to 57. Gave me about 100k total through killing and turnins. Finish up level 57 by cleaning up loose ended quests that might have arisen from this guide or your own curiosity.
Note: Just a tip for those who don't know (kind of obvious but still)...as far as gear, don't pay 50g or 100g to deck yourself out in level 48 blues. You'll enter the Outlands and quite literally replace most of your gear within a day. Yes, it is that fast. Wasting money on gear is therefore not necesarry. It was harder than I thought to avoid doing because of the habit of thinking level 60 is the limit. Be sure to realize you don't need to spend money on gear anymore (at least until you hit 70 lol).
I saw guides like this, but none really entailing a "quests that are easy experience" sort of feel. By all means, make your own gameplay, but if I had to name the easiest experience quests (top 3) in order it would be the Azshara Tablet Chain, followed by the Turnin Quest at Eastern Plaguelands, and finally the "Delivery" quests you get at the Inn in Everlook.
This was more a guide for people to reference the easiest quests to level them. Less a leveling guide then a "I'm sick of playing outside of the Burning Crusade and I'll kill myself if I don't hit 58 soon" type of concept. I hope you found it informative!
Yano
P.S. Used an undead warlock, the easiest multiple mob class in the game. Consider reading a bit into the difficulty level of some of these quests (like killing the elites in Everlook) to see where you stand as far as difficulty. I.E. I died on my 58 mage trying to solo Brumeran in Winterspring, but killed it on my 56 lock easily.