WoWDB

That's how I got to it. The key to reaching is as Aliok mentioned:

1. Turn off auto-run (default key-binding is forward-slash). The trick here is to not fall off, and using auto-run can be a great way to go splat.
2. Angle your camera so that you are looking at a top-down view of your surroundings.
3. Don't hesitate rotate your character around to double-check your position. What may look right beneath you from one perspective can turn out to be further away from a different view. (If you are uncertain, feel free to use /pray. Not only is it appropriate here, it can help with showing to what degree your character is turned.)

After going through the above, I reached the chest on my first attempt. The reward felt pitiful, in my opinion.

Nice image guide, Simca. This is much better than my accidental route and I recommend players use this instead.

TL;DR - You will need to find a way to reach Ordon Sanctuary. Run around and find a spot where you can drop down and work your way to the rope that leads to the column that has the treasure chest set on it. Don't auto-run. Take the time to correct your direction while on the rope. Be careful jump down over the column.

It will take some work and cleverness to reach the treasure chest. There may be other routes - to be honest, I probably took the long way around - but my route was actually unintentional; I'm only writing my experience.

I followed the path through The Blazing Way all the way to where the bridge is broken. I cast  Slow Fall, pointed myself at a spot on the other side that looked like I could make it to the top, got a running start and then activated  Blazing Speed (Patch 5.3 changed something so that Slow Fall cannot be cast while Blazing Speed is active...) and jumped off at the edge of the broken span. I made it and was able to mount my mammoth and get up to Ordon Sanctuary! /dance

I headed east, and, in the process of trying to reach a Firestorm Egg, fell down the mountainside and landed in front of a rope. (I bet you can see where this is going.)

The rope looked like it lead to the Rope-Bound Treasure Chest I'd spotted earlier. Lucky! Now all I had to do was cross the gap using the rope. I don't recommend using auto-run for this part. The rope itself has a texture that can deceive your eyes into thinking you are going straight when you aren't. I needed to periodically stop and correct my direction so I wouldn't fall off. There are hanging decorations on the rope; don't worry about trying to jump over where they meet the rope because there is no collision, just keep moving forward.

Now that I was standing over the stone column which the chest rested upon, I angled my camera to point straight down. This is useful for ensuring that when you jump off that you don't miss the target. I then very carefully jumped up and press the move-left button. (Even though you're jumping straight up, WoW allows for a little side movement in the air.) Even so, I nearly didn't land on the platform.

The chest contained at least a few dozen  Timeless Coin. There are probably more things that can be looted from it - RNG is RNG.