ReiserFS works extremely well on small files, so I like using it for /tmp. I would use it for /etc, /usr/share and /usr/man as well, but I've had problems where it has corrupted the FS. I can afford to lose /tmp, but /etc is another matter. If I felt safer with it, I'd consider ResierFS to be near-obligatory for any directory in which short files were the norm.
(More partitions is no big deal with gparted-style partition tables. More than 6 partitions was more of a bother under the MSDOS-style partition tables. The overheads are a pain, but in some case the space efficiency of specialist FS' in their niche will cover this. In other cases, the speed improvement is worth the space penalty, especially on modern drives.)
Btrfs is promising but the repair tools have been promised for a very long time and I'm beginning to think that Oracle might try to recover the cost of development by charging for "professional-quality tools" in the same way as is done for MOSIX. (I can sympathize with the MOSIX stance as anyone building a large cluster probably has spare cash.) Oracle has done something along these lines with MySQL and I don't trust them to not do the same anywhere they can.
ZFS has its own niche - it essentially folds hardware RAID, LVM and a logging filesystem into one unit, with tight integration and resulting efficiency gains, but being run in userspace does harm most/all of those gains. With work, though, it could be made into an excellent filesystem where you need that kind of dynamic - such as for database work, or software development. You can't predict in such cases what the final requirements will be and migration is a major headache.
The P9000 filesystem is fascinating and may well be great in certain clustered environs. There are other clustered filesystems but I'm beginning to think Lustre has lost its way, POHLFS has recently been reinvented and can't be regarded as stable yet, Gluster hasn't been heard of for a while and the "top" commercial cluster FS for Linux (Polyserve Filesystem) died when HP bought the company owning it, with suspicions of murder. The field is a bit of a mess.