When it comes to modding, the engine isn't the only factor in 'ease of use'. I've found the import and export scripts for modeling applications extremely important as well. Oblivion is awesome in this regard, because you can import and export most things to and from blender, which is free. Many engines only support 3ds or Maya. When I was researching engines for one of my projects, I was continually frustrated by this. The export scripts for Source only support static objects for blender, the X-Ray engine has no support, UDK has limited support, and I don't recall seeing any support for the CryEngine. Of course, it's been a while since I researched other engines, but unless you have a lot of money to throw at the right tools, or you have some other way to access them, most of these engines are right out. (Well, not really; Source supports XSI Mod tool, which is free, but I couldn't really get into it.
UDK might support XSI, too; not sure about that.)
Of course, these other engines have other advantages. The Hammer editor (for Source) lets you build pretty much any kind of interior environment you want with tremendous control, is easier to use than a modeling application, avoids export issues, and does its own collision. I can build a convincing interior in Hammer in about a tenth the time it would take me to build it in blender. Of course, Source isn't really built for large exterior environments like Ob/FO, so it's got it's own limitations. It's designed to make shooters, not RPGs.
If you know how to program in C++, though, you can make it do pretty much anything you want. Then again, you need to know how to program just to add NPCs, program AI, and script dialog. With Oblivion, you can literally drag and drop NPCs, AI packages, and add dialog via an editor that isn't much more complicated than Notepad. There is simply no comparison in terms of ease of use here. Oblivion wins hands down in almost every category...except when it comes to making something that isn't like Oblivion.
If you can live with the limitations imposed by the engine (and there are a lot), you can still do some pretty spectacular stuff with this game. And I don't think we've really seen it's full potential, yet, even with Nehrim.