(I'm posting this as an answer because it got a lot longer than it should have.)
SpikeX: I think you mean that Unity doesn't provide the built-in BSP-style brush-based level editing commonly used in level editors for games that support mod-ing.
I've been on a manhunt to collect as much information as possible about the process of creating levels for use in Unity and I think what I'm seeing so far is that for Unity in particular the process is more about level-design than it is about assembly. Ostensibly you could utilize any application to build your level out and convert it to something that Unity can use.
Level design as a topic is something I'm not finding a lot of information for when it pertains to Unity but I have found a few decent looking books that touch on concepts. Unfortunately most of these books rely on BSP-based editors and it seems difficult to replicate that process inside of Unity.
I've asked around in IRC and I'm hoping to get some help in finding a few points to help shape my search for more information.
The basic workflow so far appears to be:
- Sketch the level
- Block the level out using whiteboxing or basic shapes (in unity)
- Playtest your environment.
- Build level in 3d program (e.g. blender) and export pieces to unity compatible format.
- Assemble level and add props in unity.
I'd really love to get a discussion going about this topic as I think it's becoming more critical as Unity reaches mass-market saturation. Maybe a Google Wave would be more appropriate a forum for that?