Is your system scriptable todo mods or additional quests? Do you use a special engine or is it selfcoded?
The engine is 95% all me baby!
I've used some libraries like
TweenLite,
DConsole2,
Standing Wave 3,
Lua-Alchemy and
MinimalComps.
I'm gradually moving away from MinimalComps once I've done all of the menus themselves. So that's something that I'm using just for prototyping. Although if you're looking for a better prototype UI than MinimalComps get yourself
LiquifyComps. It's much, much better.
The engine itself is based mainly on XML files that contain everything that the engine uses. Be it sprites, sounds, music, animationm, NPCs, etc. Only the special NPCs (mainly bosses) are hardcoded into the game.
Atm there isn't that much modding capabilities when it comes to NPCs/AI but everything else is pretty much accessible from Lua. If you look at the 6.1 devlog with the broken door, it's done all in Lua.
I'll probably do Lua based AI at some point so when the game is released people can start to mod it as they see fit.