I am constantly surprised by how well those secret areas are hidden. The one in Lion's Arch is great.
Also the caves are fucking awesome looking.
I thought jumping puzzles would be just, well jumping puzzles, but theres other stuff in it. Like in one cave you have to light the torches and clear the mobs and then another secret underwater passage opens, it's bloody awesome. In the Lion's Arch one a spirit guides you while rambling random stuff, very funny