I think it's their endless variability and generally attractive shape. They tend to grow so well that most people tend to think favorably of them, yet there are some that are challenging enough that they provide even the most experienced keeper a run for his money. I like to think of them as the Orchids of the aquarium world.
Personally, I love Crypts because of their color and shape. I'm a big fan of lanceolate rosette plants like crypts and swords. I'm more partial to the darkness of their foliage and the subtle colors that often appear. Not to mention, they have a much prettier flower, as far as coloration goes, than do swords and they generally stay a lot smaller.