Just the basic fact that you can map the four houses to the four classical elements.

Oh! I could have sworn even JKR said that somewhere, or confirmed when someone asked the question or something like that, though I might be misremembering.

I love some of the artwork you have posted on your other LJ -- really beautiful.

Just to be clear, none of it is mine; it's all shared from dA and suchlike (with permission, or at least the tacit permission that leaving share buttons turned on gives).