There were a lot of things a developer wants to see:
API documentation; everything from, "hello world" to "AI creation" and "Where is the economic code that you'll be wanting to rip out". I'm expecting it'll be a lot like Warband: giant piles of C#, little to no proper documentation, hardcoded Gotcha stuff, etc.
I'm just hoping it's all a little more professional this time, with documentation of function calls, gamecode properly documented where needbe, and an API that allows for access to the deepest parts of the game, so that modders can, say, build a Zombie-hunting mod, where the player collects "survivors" and scours the Toxic Wastelands; no economy at all past bartering, powerful tools for storytelling, etc.. I'm not holding my breath on that.
Content workflow that covers mesh imports, material creation, rigging and asset assignment (i.e., the coding process to get your rigged armor into a game in a reasonable timeframe, explained well)
Map development, both macro and micro (i.e., the overworld and the battle / FPS scenes). Warband's in-house tools were... OK in some places, pretty poorly designed in others, and the tool crashed a lot.
Profiling tools for scene analysis, code performance, combat analysis (i.e., why is my newly-coded C# combat mechanic causing spikes? Why are framerates bottoming out when we look at this group of trees? Why is the AI pathfinding here, but not there?)
Material development; just saying, "Hey, it's PBR now" is not relevant. Modders will want to develop custom shaders, for example- some standardized PBR shader simply won't do. What is the pipeline and how do we utilize the API to make it happen?
UI development: most of what makes mods special is user interfaces. Warband actually had a pretty amazing UI system; what it lacked was any sort of good design tools, to build UIs swiftly and efficiently and test on the fly.
The list there goes on and on. They initially promised something that's kind of like Unity, but with a specific focus; C# development cycles and built-in toolsets, freedom to design animations, new character types, weapons, etc., etc., not all of the hard-coded limitations of the previous engine. Details are pretty lacking, though, which is scary at this point; they should have solid tools by now.