The X99-M WS combines the Haswell-E platform with extra certification for workstation computers, and it does so in a microATX form factor. The WS line goes through extra QA against a variety of accessories and add-in boards, which should lead to less situations where something like a user's wants to install two different video capture cards in their system, but ends up getting random blue screens.
As expected, the motherboard uses the X99 chipset, which allows Core i7 and Xeon processors with up two eighteen cores. It can accept up to 64 GB of DDR4 memory, which can be aligned in a quad-channel formation. It allows dual graphics cards from both AMD and NVIDIA, even though it has three PCIe 3.0 lanes. I assume this is because a dual-slot graphics card would cover up the third x16 bus — this is a microATX motherboard, after all.
Beyond being a small, workstation-certified motherboard, it also has USB 3.1 (which brings 10 Gbps of bandwidth to external devices). This is obviously useful for external storage, and that has a lot of uses for workstation applications.
The ASUS X99-M is available now for $279.99 MSRP. It is currently listed on Newegg at $275.99 with a three year warranty, but I cannot find an official warranty listing from ASUS to confirm that.