There seem to be quite alot of debate going on LKML over whether to include ReiserFS in kernel or not.
ReiserFS has been into problems over couple of things. Firstly, the way it was pushed by Hans Reiser was not liked by many.

Then there were talks over reliability of file system. As someone pointed out:
"The fact that reiserfs uses a single B-tree to store all of its data means that very entertaining things can happen if you lose a sector containing a high-level node in the tree.It's even more entertaining if you have image files (like initrd files) in reiserfs format stored in reiserfs, and you run the recovery program on the filesystem."

Another problem with ReiserFS is it's quest to integrate everything within filesystem. As an example it has plugins that can alter the symantics of files, like making files into directories inside which you could see meta-files like file/uid and file/size which contained meta-data and such accessible as normal files to all the unix tools. You could get things like chmod by just doing
'echo root >file/owner'.

Whether this is a good idea is quite debatable, as it is being long believed in Unix world that do one thing well and keep it simple. Next step in this direction could to parse the zip archives in kernel space for doing a 'cd linux-2.6.17.tar.bz2'(or is it already implemented) which does not sound like a good idea.
Moreover, this may require couple of changes in VFS.

I recently noticed missing readv system call in ReiserFS while calling it from kernel space.

Someone wrote an article on why ReiserFS is not included in kernel:
http://wiki.kernelnewbies.org/WhyReiser4IsNotIn

Although there seem to have been quite alot of development going on ReiserFS and it is installed as part of SUSE distributions. It seems some of the ideas implemented by this file system are unique and may be useful to other filesystems implemented in the future.