Most of the time you spend leveling will involve questing and slaying monsters, but there’s also a lot of travel. Travel earns you little experience (you do gain a bit for discovering new areas), so it’s wise to keep travel to a minimum. The new level scaling system, which scales zones to your level within a preset range, helps with that. You can choose what zones you want to experience and stick with them until you finish their quests.

-Bugs- There aren't too many glaring bugs anymore, but you will certainly encounter some. The pathetic thing is that many of the bugs you'll encounter were reported repeatedly during Alpha/Beta and still weren't fixed. Some bugs were discovered after release and players were able to abuse game mechanics without repercussion unless they were one of the more serious offenders. This gave some players a significant advantage over others because they used exploits and at the start of an expansion, that is the time when it's most impactful.

Improper leveling of your production profession skills can cost a small fortune. Heck, even proper leveling of some production skills can cost a small fortune. And keep in mind that equipment you produce using your profession will typically be slightly worse than equipment otherwise obtainable at your level via the Auction House and/or instances. It is therefore strongly recommended not to take on a production trade skill until you hit at least level 30, or better yet, level 70+. However, if you are determined to take on such a profession (particularly under level 30), read a suitable leveling guide in order to gain whatever skill level you desire for the least amount of money.
Professions are skills used to create and enhance items (gears). You will start with one skill once your character has learned a profession. When you use the profession, your skill will increase. A profession’s higher skill will allow you to create more powerful items. If you change a primary profession, you will lose any skill you have made with your original profession. So it is important to choose your primary professions wisely.
For the duration of the fight, creatures in the immediate area will not be visible to you (but they may still attack you if they wander too close). Keep in mind that other players will still be able to see you and attack you if you are eligible for player vs. player (PVP). Other players will also be able to view your pet battle in real time, including the pets involved as well as their health as the match progresses.

The most simple thing is TradeSkillMaster. This AddOn shows you quite an amount of information in the items tooltip. In my opinion, DBRegionMarketAvg the best source. It tells me the price, an item was posted on all EU realms in the last 14 days. Why not DBMarket? DBMarket only shows you the items value from your server in the last 14 days. This value often gets manipulated. Some people start to post an item with a 500g value for 500.000 Gold on the auction house for several weaks. As a consequence, DBMarket is rising towards 500.000 Gold.  If you use DBMarket with 100% and post your item for 500.000 Gold, nobody will buy it.