There's been dozens of Phoronix articles already covering the features most interesting to Linux 4.0 and other prominent changes while in this article is a fresh recap -- given discovering other changes and my experiences with benchmarking Linux 4.0 thus far.
Live kernel patching is surely one of the most advertised features of Linux 4.0, that comes as the joint work done by Red Hat on Kpatch and SUSE on kGraft. With Linux 4.0 the initial infrastructure is in place but look for more work to land over the later Linux 4.x releases.
For the favorite Phoronix area, the DRM/graphics drivers, there's the usual improvements. The Radeon DRM driver now has DisplayPort audio support and better fan control support. The Radeon DRM's sister driver, AMDKFD for HSA support, started working on Carrizo APU support. When it comes to Intel graphics, there's changes all over but most important for end-users is likely the Intel Skylake support taking shape. There's also basic Nouveau work and other DRM improvements for Linux 4.0.
Another favorite subsystem comes down to file-systems / block devices and for Linux 4.0 we have pNFS block server support, Btrfs RAID 5/6 improvements, and new OverlayFS functionality.
Other interesting Linux 4.0 kernel merges include Intel Quark SoC support, new ARM hardware support, IBM z13 support, better Toshiba laptop support, improved Logitech HID support, and a smuttering of other internal kernel work.
Linux 4.0 also comes with an odd codename. This release was formerly known as Linux 3.20 until Linus Torvalds decided to go ahead and make it v4.0. Stay tuned for more Linux 4.0 testing in April on Phoronix.