1. An all-in-one editor, so it is not necessary to open many different programs to get at all the functions.
2. Everything in the game editable.
Myth 2: Soulblighter, if anyone has played it or edited for it, did a very good job at having the entire game editable in its editors, but the only problem was there were two editors, Fear and Loathing.
However, between those two editors, EVERYTHING in the game was editable, from unit appearances, to unit scaling, to unit naming/unit texts, to level elevation, splash size, color map (not made with pregenerated textures), to unit attacks, projectile statistics, gravity amounts... basically anything imagineable. For example, without even importing any new resources (bitmaps, etc.) it was possible (In Myth:2) to make archers shoot flaming pigs which exploded in a massive gout of blood, caught the ground on fire, and steamed when landed in water. If you had the time and the software, you could even create entirely new units, and not just edit the textures of existing ones. And it was even a Real Time Strategy game... The only problem was the files from the editor could not be downloaded within the multiplayer game (most likely because most files that included completely new color maps were larger than 5 MB). It would be nice to be able to do this in every game editor (without the massive file size thing).