I'm still much involved in storage so I'll start from this part of the system, The two biggest novelties here are the latest and greatest version of ZFS (v28 as of this writing) and UFS-SUJ.
From the ZFS front we expect fancy new features (compared to v14 which is the latest in STABLE) like deduplication, log device removal, RAID-Z3, user and group quotas, and a large number of performance and stability improvements. It's been a couple of years since ZFS was first imported in FreeBSD and it's been a text-book case of how even when the APIs are matched there remain a lot of semantic differences which lead to behaviour from crashes (like the early memory size crashes) to slowdowns (though some of those were not FreeBSD's fault). Hopefully, 9.0 will finally ship with ZFS in which the edge cases of the integration are smoothed enough for serious production use. It is likely that 8-STABLE will stay with ZFS v15 MFC-ed (with the majority of the stability improvements). Major developers working on ZFS for 9.0 were Pawel Jakub Dawidek, Martin Matuska, Andriy Gapon and Kip Macy.
The venerable UFS has finally received integrated journalling. Earlier it was possible to use gjournal (in whose making I've actually participated in an roundabout way) but it was a bulk-data journalling effort, making no distinction between data and metadata, requiring a large journal. The SUJ mode is an addon to SoftUpdates, extending it to record a very small intent-log journal for the edge-cases where it required a (light version of) fsck. This development makes UFS a fully fsck-less file system in the common case. Users of UFS quotas will be happy to finally have 64-bit quota counters, allowing more than 2 TB quota rules, and security-conscious will probably soon endorse NFSv4-style ACL security rules. Default tuning for the UFS has also changed a bit (by me), making it significantly faster with devices which support tagged queueing (like SATA with AHCI) and where disk access latencies are high (like in virtualized environment). Maybe we will finally see decent results from third party file system benchmarks with default tuning. Major developers working on UFS were Jeff Roberson, Kirk McKusick and Edward Tomasz Napierala.
As mentioned before, the new AHCI driver for SATA is stable and offers better performance so is recommended to everyone with SATA drives. Its developer is Alexandar Motin.
Users requiring highly available (replicated) storage will make use of the HAST daemon, a userland GEOM replication system by Pawel Jakub Dawidek.
On the general development side, LLVM support is progressing nicely and it was imported in the base system. It is still undecided if it will be the default compiler for 9.0, but it has proven itself to work correctly. The LLVM/CLANG effort was spearheaded by Roman Divacky, Pawel Worach, Ed Schouten, Rene Ladan and Dimitry Andric.
Rounding up the DTrace import is the official support for tracing userland processes with DTrace hooks. This type of tracing has already been accepted in some large projects like PostgreSQL and X.Org for internal testing and debugging, and offering the feature in FreeBSD makes it more accessible and gives incentive to spread it to more projects. The implementor of userland DTrace was Rui Paulo.
Revamp of the timer infrastructure is progressing nicely, and will probably soon enable running a "tickless" kernel (without periodic timers if they are unused). It is developed by Alexander Motin.
The VM system and superpages received performance optimizations by Alan Cox and other developers, and first steps toward a NUMA-aware system were made by John Baldwin.