Though I still haven’t answered all my questions (e.g. what Xegl will have/Xgl has what AIGLX is not providing already) I found a very interesting page which gives understandable explanations: Communication between Xorg, Xgl, and an OpenGL client, through libGL and the GLX Protocol. It is focussed on Xgl but you will find explanatory themes for AIGLX as well – and these show by the way very good why Xgl is in a way a hack while AIGLX is very integrated into the current X.Org structure.
And I really like the way how this page explains the communications process between for example Xgl and Mesa:
Mesa libGL: “Hello Xgl GLX Extension. I need an OpenGL Context for my client (that’s Compiz).”
Xgl GLX Extension: “Sure, I’m giving an Indirect OpenGL Context to you. Now you can use OpenGL functions through the GLX Protocol.”
That can be understand by everyone.