I ran out of fishing baubles so I decided to take a better look at the place around the inn in Dalaran, maybe I find another Underbelly Elixir or something else.
That's how I came to find a way how to reach both sides of the Sewer's Inn, without using lag or the transformation from the Elixir mentioned above.
Just follow the steps, there's a harder way which requires you to jump between the wall and wall of the inn, but you often get stuck with your head when jumping on the crates so this one is a lot easier.
(Sorry for the file size, I thought resizing to 800x600 but quality is awful.)
Climb up the stone wall, from there you can reach the rear of the boxes.
Once on the wall, take the path left to reach the first box.
Jump on the box.
Jump on the bigger box and prepare to jump on the little wooden ledge.
Jump on the other wooden ledge.
Jump on the red thing.
Mount up and jump to the other.
From the second red thing, jump to the third wooden ledge.
From there just jump to the top, voila.
Giorgio Armani, a.k.a Remah. (copyright at MMOwned - first poster)