Intel has some awesome new NVM Express SSDs for the PC. The 750 Series gets 2400MB/s read and 1200MB/s write.
Another change in 10.10.3 is massively improved support for 3rd party 4k monitors. If you look at Apple's 4K support page, they used to list a small set of supported monitors, but now they just say with OS X Yosemite v10.10.3 "most" 4K SST and MST monitors are supported.
Edit: also, something else I just noticed on the...