Usually it's best to avoid a lot of instance runs. It is too difficult to account for bad groups, and some instances are too far away with too few quests. Since the experience is very good, it's a good idea to clear nearby instances which have a lot of quests once (Gnomeregan, Deadmines for Alliance). Distant instances with few quests should be avoided (Deadmines for Horde). Low level instances can be cleared by paying or befriending a single high level character. However, since patch 3.3 a new Dungeon group finding system has been implemented, as well as a direct port to dungeons (when entering the group you get ported to the dungeon and back to the same location if you leave it) and thus the travel time to dungeons has been greatly reduced. This makes it a good idea to complete quests or grind while you wait for a group to become available.
As it goes with almost any MMORPG, World of Warcraft is a very grindy game. In it, you’ll have to pump your character to the maximum available level before you can get to the end-game content which all players are so used to. And the problem is – the process of going through all the expansions just to get yourself one new class to be playable in raids is very time consuming. Even if you have a vanilla WoW leveling guide at your disposal, you still have to do all the boring quests or clear those repetitive dungeons over and over again. This pattern affects almost every activity in this online world. At the end of your day, you need to really dedicate your time to playing the game or you will get nowhere.