I like to routinely go back to older kernel versions. I've seen some disheartening performance on some of my machines as I upgrade. Something I've noted subconsciously for a while is Reiser slowing down. Then after browsing around I see the following.


http://kernelnewbies.org/Linux_2_6_33

1.13. Reiserfs de-BKLification

One of the biggest shortcomings of reiserfs v3 (and one of the reasons why most distros use Ext instead) is that its codebase handles concurrency using a single big lock - the BKL (Big Kernel Lock). This means that its SMP scalability is very poor. This release won't fix that issue, but it replaces the BKL with a reiserfs-specific solution. In this release, there are no more traces of the BKL inside reiserfs. It has been converted into a recursive mutex. This sounds dirty but plugging a traditional lock into reiserfs would involve a deeper rewrite as the reiserfs architecture is based on the ugly big kernel lock rules.
Due to the subtle semantics of the locking changes, some workloads may have small performance regressions and other have improvements.

Has anyone noticed other problem areas to look out for?

Personally, no, not really, but if you look at Phoronix they do track performance regressions in various kernels (amongst other things).