It has been said that if you bring in more than one quest at a time to turn in and do so without starting another one or killing anything between them, that you will get a bonus to your exp. So you might try gathering several up at once and then making one run to turn in several at one time. I know that I have been advised of this over and over.

I've never been made to believe that there's a reason to do this as far as the game mechanics are concerned, but, especially if it's your second or third...or fourth character, and you know the zone, try to plan out where you're going to quest so that you can basically just do a circuit of the areas you need very quickly. So, while I don't know about any bonus to XP, I know that the travel time you save doing this really speeds up the process.

Here are some tips from your friendly neighborhood pyromaniac:
*If you can easily kill it, and it aggros onto you, just kill it. XP is XP, and otherwise those walks from point A to point B would be relatively fruitless. Plus, if you run from too many mobs, it'll get you killed enough of the time that it's not worth it.

*For the most part, questing with a friend is a super great idea and it dramatically speeds up the process *and* increases the quality of your playtime, but it doesn't work for some quests. Quests where you have to kill stuff and everyone gets credit are good to do with friends. Quests where you have to collect a bunch of stuff that mobs drop can be very frustrating to do alone, and even moreso with someone else to share the drops with. Any of you who have 70s know what I'm talking about. (Fucking Ore Chest keys...)

*Orange quests and mobs are never, never, never worth it, unless you're in a group, and even then they're only good if you're at that awkward stage between 45 and 50 and you've run out of good quests. Green quests are good but not worth your time if there is a yellow nearby that you could do just as easily.

*If you're invited by a good group to run an instance appropriate to your level and you know you don't need to leave within the next hour or so, go ahead and do it. Even if you don't have quests for them, instances are better for your xp/hr and your purse than solo leveling could ever be.

*Always, always, ALWAYS log out in an inn or a capital. ALWAYS. There is no reason not to do this.

*This isn't a leveling thing, but you'll thank me when it comes time to get your mount. Crafting professions are evil. The auction house is evil. ONLY craft with cheap vendor bought materials and what you find, and ONLY go to the auction house to sell things, until you hit 70. Believe me, having a mount is way way nicer than anything you can make crafting at level 40 or buy off the AH.