The four theme types I've come across are the icon, L, Today, and Zen that cane with the pearl. Are there any other types available, or are all the extra themes just variations on those basic ones?

I'm not sure what I'm looking for, I just wonder what's out there, and what changes with the themes besides icons and color schemes.

I believe those are the basic categories, with theme designers making their customizations based off of those. I don't know that I have seen any other "types" other than the ones you have mentioned. The L theme is really kind of based off the Zen style, too. It combines the regular Zen theme with the bottom-docked-icon Zen theme.

Another I've come across on the inter-web here is the Today Plus, which seems to be the today theme, with some icons across the bottom. Anybody know where I might find a free download of this one to try?