First off I'd just like to introduce myself. I'm Gwapesyo of Archimonde - US. I play a Priest currently shadow, though I've played disc in the past and plan on playing it again in the near future. Moving on, this article will be about my opinions on leveling a new class, how to do it and why.
First things first. You might play a class now that you've played for a long time and are getting bored of. So you think to yourself, I want to play another class! Well I think that you should do that then. You obviously enjoy playing this game, so if the class your playing is keeping you from enjoying it, reroll.
So now that you've picked the class you want to play, you need to start thinking about what spec you want to play at level 85. Lets say you're leveling a priest. You want to be a healer in arenas at 85. So now you look at all 3 specs, there's Discipline, Holy, and Shadow. Discipline and Holy being your two healing specs. After doing some research you find that you want to play Discipline. Spec discipline while leveling, do dungeons and even queue bg's throughout your time leveling. This will help you learn that spec long before you hit 85 so when you do finally get 85, you wont have to respec just to find out that you have no idea how that spec works. Yes leveling as a healing spec might suck, but with dungeons it is doable. Also now you can have dual spec, so you can grind quests between queues as shadow and still be able to heal dungeons or bg's.
Next I'd like to talk about fast ways to level. Leveling fast is going to depend on what class and spec you pick. First I'll discuss pure DPS classes. For this we have Rogue, Hunter, Mage, and Warlocks. These classes have no option of healing or tanking. Therefore the queues for dungeons will be much longer and makes dungeon grinding less viable. My recommendation is to queue for dungeons while grinding every quest an area has to offer. Since you won’t get queue's to pop for around 20+ minutes, this will give you plenty of time to do and turn in many quests between queue's.
Now lets talk about Tanking classes. Here we have Warrior, Paladin, Druid, and Death Knights. These classes can tank dungeons which makes leveling through dungeon grinding more possible. I'm currently leveling a warrior and tanking dungeons with instant queue pops. In about 1.6 days played time I am level 40. I did quests to get to level 15, and went straight to queuing as a tank for random dungeons. This method is very effective.
Now the healers. Paladins, Druids, Shamans, and Priests. These classes are almost the same as tanks, with fast queues. However, its not AS fast. I recommend level'ing the same as you would any DPS class. Or even find a tank and queue as tank and heals for instant queues.
I hope you liked what you read and that this wasnt a big wall of boring text for you.
I will absolutely try to make this more interesting while still being informative. Please give me feedback on this.