While some may think the inclusion of a level editor may not be that big, just look at the thing! Now there's some slick integration with very accessible user controls really anybody can build levels with. It really makes me wish more games had such a feature. Tinkering around in an engine editor isn't the idea of fun to me. Just thinking how great a tool something like this might be for regular arena style FPS. Halo's Forge is about the closest thing we got before it. It wouldn't surprise me if Valve decided to include a similar editor in some of their future games. It sure would make sense.

Also, just for entertainment: Valve's promo reel for the editor.

And in good Valve troll fashion they hid a small Easter-egg in there for something latter on. In this case it's the Meet the Pyro video. At least it wasn't a reference to HL again.