Yea, but so far, there only one game seems to /really/ get how to make a map editor. Thats Halo: Reach, its Forge mode is unparalleled in capability, especially for a console map editor. I wouldn't be shocked, if Valve manage to pull off something like it, but I would be greatly impressed. Its not an easy thing to create.
I dunno. As someone who has played Warcraft 3 (and used the map editor extensively with that), Starcraft 2 (and got fed up with the BS Battle.net 2.0), and many Bethesda games (new site for every one that comes out, for mods alone), at least Bethesda and Blizzard know how to make an editor.
In Bethesda's case, it's practically the same thing as they use to make the game itself, but still.
I know that Bethesda games get highly modded, but I wouldnt really call that map editor.
Now Blizards Map editors are fantastic, but, it's not as far reaching Forge mode is, I would argue. Forge in Reach, really lets you create unique geomtries, and maps (though phasing, being able to place game item anywhere), whereas in WC3 or SC2, you're stuck with its geometry, which drastically limits what you can do within.
The most notable thing that was achieved with those editors, and this is including previous Blizard title, is the popularization, and creation of the modern Tower Defense sub genre. Which, is /huge/, dont get me wrong, but with the Forge mode, new games have been made within.
Racing games have been made in Reach (with more first party support), Challenge Maps (Achieve Hunter's Horse videos), Griff Ball, and Forge Art.
I am aware of Art maps in SC1 and WC3 editors, but come on, this is a step way above that.
Also, the Source SDK, isn't a map editor, thats a game engine. Of course a game engine is going to be superior to a map editor. It has Software Development Kit in its name.