Getting Started - A Basic Checklist
Not a lot of guides up yet so here's my take. I've had the opportunity to level 2 hunters, a warrior, a mage, a priest and a druid past level 20. So I thought I would share my experiences. Also from speaking with others in the game it's amazing how many of the little things escape us. Here's a checklist of things that are good to know, (some of it Alliance specific):

1. Hearthstone. Everyone has one in their inventory when they start. It's basically a bind stone. By speaking to an innkeeper, you can bind to that inn. Right clicking on the hearthstone will port you back to the last bound spot. The tooltip over the hearthstone will tell you where you are bound.

2. Inns & Resting. Whenever you step into an inn or a major city, your level counter is replaced by zzz's. And you will get the message "You feel rested". Basically, the longer you stay in the 'zzz' mode, the more rested you are. Your xp bar turns blue, and you should see a little pointer on the xp bar. All fighting experience you acquire up until you reach that pointer will be doubled. About to complete a quest? Don't worry, the pointer will automatically be adjusted so you can still get the double xp from fighting. And the best part? You can log off while parked in an inn and that pointer just keeps on moving, up to a max of 1.5 levels. I tend to alternate characters, so I basically always hunt with double xp bonus.

3. Food & Drink. You can eat and drink at the same time. Cooking, a secondary skill, will allow you to make food and drink that also has special buff bonuses.

4. Alchemy & Potions. If you can, pick up alchemy and herbalism. Early on you will be able to make instant health potions that can be used while in combat. Also, you will be able to make helpful buffs early on, buffs that last 60 minutes (provided death does not come first). Other potions include water breathing and increased swim speed.

5. Weapon Skills. All classes start out knowing a handful of weapon skills. There are weapon trainers in the major cities that will teach you additional weapon skills for 10 silver (each city has unique offerings). Night Elf hunters, for example, can journey to Ironforge to learn guns. Warriors can learn maces and 2handed mace, etc. Leveling a new skill goes very quickly, and you can alternate weapons as you level to keep your skills up. Keeping unarmed skill up is helpful for fighting mobs that disarm you.

6. Getting to Ironforge from Night Elf lands and back. This question gets asked in general chat over and over again. Basically, from Darnassus you port to coastal town. Here you can take bird or boat to Auberdine in Darkshore (which is located on Horde continent). From Auberdine take boat to Menethil Harbor. If it's your first time, you'll then need to run through the wetlands (20+ mobs), to Loch Modan, then Dun Morogh. Remember to speak to the griffon masters in Menethil and Ironforge so you don't have to do that long run ever again!

7. Getting from Stormwind to Booty Bay for Fishing Trainer. I tried running it at level 20 and what a nightmare that was. Then someone suggested swimming from Westfall along the coast to Booty Bay. Aside from one nasty rendez-vous with a Murloc, I made it! Got my fishing book, found the griffon master and flew back to Stormwind.

8. Crafting Mules. My strategy is to have my crafters parked in the major cities, while my adventuring characters pick up the collecting professions. Then I just mail the raw materials to my crafters. This works great, except occasionally i do need to go xp my crafters since they need level 10 for expert and level 20 for artisan.

9. Auction House. The only auction house is in Ironforge. I park one of my level 5 characters at the auction house full time. I use him to buy and sell on the auction house, using the mail system to move the items and money to my other characters. In my personal opinion the auction house is an awesome place to find and sell items.

10. Mail System. Probably one of the best features WoW has to offer. I use the mail system to transfer items between characters; leave messages for offline friends, help new friends get started. Most of my characters are no longer anywhere near the beginner zones. When new friend joins the game, I email them a couple of crafted bags, some getting started hints, a weapon and some armor. I don't have to roll a new character to join them and yet they still feel I'm helping out!

11. Guards. In all the major cities and some of the smaller ones, you can right click on any guard and ask for directions to trainers, merchants, banks, etc.!

12. First Hit gets the XP. First person to engage a creature in pve combat will get the experience for the kill. The background color of the targeted mob title bar will show the status of the mob. A gray background means that the mob has already been engaged by someone else.

13. Looting Schemes. Still some confusion here. Round Robin is each person gets a turn. Free for all is first come first serve cut throat looting! Group loot is basically Free for All with rolling for special items. Need before Greed is Round Robin looting with rolling for special items (so it was explained to me).

14. Power leveling and Aggro management. I tried to powerlevel a level 12 priest with my level 24 hunter in Darkshire (20+ mobs). It was nasty. Aggro range is directly proportional to relative level - the greater the difference in level, the greater the aggro range. A level 20 wolf will aggro a level 12 priest from an insane distance, while a level 12 wolf will only aggro a level 20 priest if the priest is practically right on top of the wolf. Best way to Powerlevel was to let the level 12 priest get the first hit (ungrouped) and then my level 24 hunter finish it off. Gets boring fast if you're the hunter :P

15. Talents. This becomes active at level 10. You get ONE talent point per level starting at level 10. This allows you to specialize in 3 different schools, based on your playing style. If you mess up or have a change of heart later on, you can visit your trainer and (for a fee) get your points back.

There's probably more things to note and everyone makes mistakes so please feel free to comment and build upon this. Happy hunting!