The mod and map editore is going to be like Warcraft 3, only with even more power and flexibility.
SC2 editor is going to be shipped essentially with a full blown compiler and it is no longer just a "map making tool," but rather a "game development tool", since you can insert entire new races into the tech tree and have them show up in the GUI in the loading splash screen, and even add them to drop down menus. Karune has already said all this in one of the Q&A batches.
Additionally, you will be able to write spell/ability scripts for SC2 directly, instead of doing "work arounds" with dummy abilities and triggers like in Warcraft 3.
In other words, in warcraft 3, you could edit existing abilities, and make "clones" of those, but it was very tedious to make all new abilities. (had to use a dummy ability and write triggers to detect it being used, etc.)
In SC2, you can, in theory, create all new "spell/upgrade/ability" scripts from scratch.
With the tech tree being very easily editable, like Warcraft 3, and the ability to add/remove weapons and abilities from units without the need for a 3rd party hack, this means you will be able to create TRULY remarkable "historical" games: Civilization, Axis and Allies, Diplomacy games, with each race having totally unique version of terran tech.
No longer will UMS games need to "peg" one unit and replace it with another. The tech tree will be seamless, and because 1000 triggers wont be running in the background, games will actually play faster and smoother than in BW UMS games...
As an example, in the UMS map "Diplomacy Gold 7.0", each "human" race in Europe gets different versions of infantry, tanks, and aircraft, and they are all very IMBA because one group gets a marine, another gets a hydralisk, another gets a ghost, etc. The thing is, they all have completely different damage types that just don't end up being fair.
Well in a SC2 version of that map, everyones "marine" would have a "marine" skin, and just have slightly different stats: The nation that is good vs mech might get +1 vs armored, or the race that is good vs infantry +1 vs biological, and the demolitionist race might get +1 vs massive, etc.
So the Starcraft 2 game engine and editor will make for VERY powerful modding potential, unlike ANYTHING anyone has ever seen.
They have also implemented the ability to add custom resources and have it show up in the GUI and HUD just like minerals, gas, and food currently show up.
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It is possible that you may even be able to implement your own damage type modifiers, though this is just a guess right now. i.e. +x vs zerg, +y vs terran, +z vs protoss, +a vs custom race1, +b vs custom race 2, etc. (hopefully this would be in there.)
If this is not in the editor, then it would be the one thing I can think of that I WOULD suggest that I don't already "know" to be in there...