I also watched the live stream and I can't wait for it! Finally a developer caring about modders and wanting to make a long living title. A game being free and not f2p is also a whole new concept for AAA titles and a steam marketplace-like system that allows modders to earn money from their creations is also a great idea (free mods can still be published of course). I also don't mind paying the Unreal Engine 4 (unlike UDK which is free) as it's subscription based, but you can keep the last version you paid forever, but recieve no updates. So you can subscribe for one month and pay the 20€, downlaod the whole package including the source code and then unsubscribe and keep it. You only have to resubscribe if you want to recieve the latest updates, but you could do that like only once a year.
All in all I still can't belive Epic Games is actually going to do this. There are taking so many risks in developing a game they way they are going to do it and I think that alone is great, but if the game and community will develop the way they planned its going to be truly UNREAL!
Edit: it being PC only and developed for mouse/keyboard makes it better than UT3 already