There’re many reasons to go back to the main city during your grinding, but I guarantee you most of them can be done with a lvl 1 char sitting in the main city, just to check the AH and buy stuff for you. You mail some gold to it to do your dirty work, and it can mail the items back to you. I am sure every one want to wow power leveling fast in the game. You can always take these quests and do them if and when you have time while you are power leveling - but you don't have to do them, unless they are a specific special quest for your toon to level or gain skills.

 

Keep that in mind if you really want to power leveling fast. Getting into big discussions/chats in the /gchat or /wchat or whatever is always fun, and it definitely makes the grinding less boring, however it will slow you down. You can go from 40k/hour to 30k/hour just by having a conversation in the chat which doesn’t seem to take up a lot of time.

 

If you go back to IF/Orgrimmar every half lvl to check AH, you will lose a lot of time with the travelling. Although you may have your hearthstone there, the trip back again will cost you a lot of time. You want to check out which quests will have you grinding to kill several of a specific mobs or will provide you with easy experience simply for being somewhere. There will be quests that won't be good for power leveling, such as those that take a long time and offer little expreience points to you.