1. For user-friendlyness try to look at games like little big planet, Mario Maker and Disney Infinity. These tools are made so simple a kid could use them. I'm not saying that the editor should be simplified to these extremes, but it's a nice source of inspiration when idiot-proofing the editor.

2. I also suggest an official mod manager. There is nothing that inspires mod makers more than seeing their mods being played by the community. An official mod manager with community updates, rating systems, download client and load order management would help bringing modding to the masses. For inpiration check out the nexus mod manager http://www.nexusmods.com/skyrim/mods/modmanager/? or mod organizer http://www.nexusmods.com/skyrim/mods/1334/? for skyrim.

3. An interactive beginner tutorial in the editor itself, where you get to make your own quest mod through objectives. It think it is key that this feature allows creativity by the user. For instance you start out by being asked to make a house with at least 4 wall, two chair, a table and a door. Then you make an npcs who with some dialog options, thats sends the npc to kill three creatures of their choice.. and so on... The important thing is that the player feel they are making something unique even though they are following a tutorial.

Thanks for Being so awesome to the community NorD!