Thanks for starting up this discussion Bischofftep, because I think it is a vital one if Blender ever hopes to become a serious (production level) 3D suite. I am absolutely for integrating the ability to render in passes, and some of my thoughts on the issue can be seen on this thread on ambient occlusion by Little_Cube. Anyone interested in multi-pass rendering should read this thread for some basic concepts.
Firstly the terms multi-layer and multi-pass rendering are sometimes used interchangeably. I like to think of multi-layer rendering as rendering the scene in parts with different objects in different layers and then compositing them in post. This itself is a very useful technique to have total control over your results, no matter how frowned upon it seems to be by amateur 3D ‘purists’ - the fact is all professionals use compositing and post-processing. Your render is not sacred - live with it.
This can already be achieved to some extent using Blender’s layers, but I think any further tweaks to the code/interface to better facilitate this kind of work flow is always welcome.
Multi-pass rendering involves rendering the various visual attributes of an entire scene in passes, and then compositing them together to get your final result. The resons this is done, include speed, better control over the results, and the reusability of data, amongst others. This is much needed in my opinion, and it is also the technique which works best when it is supported and built into the software. The different passes can be manually created with settings, but that would defeat the purpose of a smooth work flow.
I’m including some links here. I’m sure there are many who don’t totally understand the significance of this technique, and there are others who would like to know more about it. I hope this list of sites provides enough raw material for every level of interest.
Introductions to multi-pass rendering
How MAYA handles multi-pass rendering
Multi-pass rendering in XSI
This is a must see for those who would like to work on this feature. While I have never used XSI, from the description and screen shots, the XSI interface to handle multiple passes seems to be the best I have come across - a simple menu to create passes based on standard models (diffuse, occlusion etc), user defined settings, and even layers (thus integrating multi-layer and multi-pass rendering under a single interface). This is exactly the sort of thing Blender should aim for.
Multi-pass Rendering Surface Setup For Lightwave 7.5(and Maya)
A good tabulated description of the settings and features of the different passes. This should be useful for anyone wanting to work on this feature, and also for anyone wanting to fake the passes with Blender settings until (hopefully) we get native support through the interface.
A step-by-step practical guide to how a multi-pass image is put together.
I hope these links are, in the least, good starting points for understanding this subject. It is true, as some have mentioned, that this feature will not be of interest to the casual dabbler/hobbyist. However, I think Blender has already gone way beyond being just a tool for dabblers in CG; it was always meant to be a production tool. As long as the multi-pass features are invisible to anyone who doesn’t need them, I don’t think it should be an issue. This is a feature that is essential if we want the majority of professional users to not look at Blender as something you play with before you switch to Maya for real work. I myself wish to use Blender forever, and never want to switch to anything, irrespective of the importance or complexity of the project I am working with. For that attitude to be duplicated on a mass scale, multi-pass rendering is a must.