We had to stop adding things to the free version because the underlying structures just couldn't take it anymore... So the paid version represents what we've wanted to do with the game's design: There's an overarching metagame, quests, better player self-determination, a Kingdom you build up over time, preparations that you can take into dungeons, way more dungeons to find, re-balanced classes/races/gods, etc.
The Special Edition exists simply to support development: It's a way for players that really enjoy the game to say thanks and keep us going until the game's finished. They get a limited edition version of the game (we won't have the special edition up for sale after release) with an extra class and building. Note that the extra content isn't needed to get anywhere in the game, it's all bonus. It's so bonus that it doesn't exist in the beta as yet.
If you're enjoying the free version and want to expand what you're playing, then I'd suggest picking up the $10 normal version (which will be $15 when complete) - or if you're not sure, keep playing the free version and wait for the game to be complete so you can take a look at the demo