It shouldn't be drafty if you have proper weather stripping around it. I would love to have one to get extra light in the front hallway. Stained glass would be great too!
As for how hard it would be to reinstall, that all depends how they plugged it up. If they just put a plywood over the hole and some drywall, then it will be pretty easy (just rip out the crap, patch holes, and reinstall the window) If they butchered it, then you might need to rebuild several frame pieces. I think it will still be time consuming either way (you could put a temporary vapour barrier over the open hole while you're working on it).
The only way to find out is to open up the hole and have a look. If it looks butchered/terrible, then just re-patch it.