Few month ago I saw a big pdf-manual of all LaTeX3-packages and the new syntax. I think it was bigger than 300 pages. I can't find it on the web.
Does anyone have a link?
You are probably looking for the documented sources of the l3kernel package(s).
What you probably have seen was a typeset version of all the l3kernel commands and their implementation (which actually is more than 500 pages) a document named
Beside the kernel documentation you will find additional material in l3packages and l3experimental. As the name indicates "l3experimental" contains material that we are experimenting with. It should be stable in the sense that we believe it is working properly, but we are evaluating aspects of user interface, functionality etc. Feedback is therefore very welcome.
(We also have some l3trial bundle, but that is not placed on CTAN as it contains direct work in progress --- for those interested you can see that stuff in our svn source or its github mirror --- again feedback and suggestions are welcome, but please no "xyz doesn't seem to work ..." that certainly can be the case in that area.)