There's an small issue with the compatibility of the case and the motherboard. "ASRock B75M-DGS Micro ATX LGA1155 Motherboard has an onboard USB 3.0 header, but the Cooler Master HAF 912 ATX Mid Tower Case does not have front panel USB 3.0 ports."
Is this a big deal, or is there a work around for this?
I would say "not a big deal". All it really means is that the front ports on the case will run at USB2.0 speeds. There are still 2x USB3.0 ports in the back (as well as 4 more USB2.0)
If you have no USB 3.0 drives (you probably don't) this doesn't affect you at all. If you happen to have an external USB3 drive, you just plug it in back as normal. There's really no point in having USB3 on the front these days just yet. Even if you have a USB3 flash drive, they're still small enough that there's really nothing that having USB3 on the front benefits from. You'd be cutting down a 2 minute transfer to like... 1 minute.
If it IS a problem for you to have USB3.0 on the front, you can either
1) Get a different case or
2) Get a USB3.0 front panel header to go in the floppy or other CD drive slots.
Not a deal breaker in my opinion, My case has USB 3.0 ports but my motherboard (P67 Chipset) doesn't have any onboard USB 3.0 motherboard headers, I only have 1 USB 3.0 drive and I just plug it in the back if and when I use it.
If you like the case you've picked go for it, as already mentioned adding some USB 3.0 headers in a drive bay can be done easily later on.