Impressive due to its power (Dettifoss)
I find Dettifoss very impressive due to its power, mass and energy. I guess the grey water color empathizes this impression even more. At home, you can get an idea when watching http://youtu.be/zRPmfhvJuoM in full-size and in HD quality, but be warned, it's really powerful ;-)
I'm quite happy not to have seen Dettifoss when it's in best shape -- it can grow MUCH bigger according to the guide. Imagine all the water that comes down when a volcano erupts below the glacier (called "glacier burst"). As you can see in the photos, the water level is not nearly at it's maximum height. And as you experience while walking from the parking towards Dettifoss, usually only one of the river beds is in use, but during a glacier burst, other beds are also used so we'd have a group of several waterfalls more or less as big as Dettifoss. Wow!