Some players even turn this into their profession by systematically "plundering" vendors in the game world and then selling the items on the auction house at a significant markup. The reason why this works (even for items which are on unlimited supply at vendors) is, that many players don't want to spend time traveling to specific vendors to get hold of a recipe or skill book. They would rather pay a slightly higher price at their local auction house. In some sense, they use the auction house as a "super market" or "convenience store". So it is completely reasonable and legitimate to be the supplier for this convenience store and make money out of it.
The waypoint directional system helps you to easily spot and reach a quest NPC or target. The step-by-step guidelines give you an idea of the materials you will at later stages. This minimises the time wasted while backtracking. Your quest progress and level will be updated automatically by the guide. This feature makes it easy to use any character. If you want to take a break or switch to battlegrounds or dungeons, you only need to re-tweak the guide to match your need at hand. Zygor Guides also come with flight path usage, turn in and quest accepting.
Then 8.0 came along and turned everything on its head. Most of the techniques and shortcuts discovered, after 7.3.5 introduced scaling, were nerfed into oblivion. Many of the old techniques were confirmed by myself, first hand, to no longer work. I'm hoping that this thread can serve a similar purpose as the old thread, and gather as much data as possible on the fastest methods under the new system, and facilitate as many different perspectives and opinions on speed leveling with constructive discussion. I've just finished leveling to 110 under 8.1 with the exp curve.