I have had my D54250WYK for 4 months now and no problems. The fan looks to be a $10 item that could be swapped out by removing 2 screws and pulling an electrical plug. Problem is where do you get a replacement fan? As far as I know the only fix is to return the whole kit.
I have BIOS 18 installed. Intel recommends a "Minimum Duty Cycle setting of 40. At that setting my fan RPM was 3800. I dropped the MDC down to 24 and accepted an idle CPU temp of 46C and fan speed of 1950.
No problems after three months at latest settings.
I have to have my ear within 2 feet of the NUC before I hear the fan and then it sounds like White Noise.
I intend to buy another NUC when The Broadwell chip comes out.
From Intel: "We have seen this noise issue before and it has been escalated to our engineers. They are trying to find a solution for this. Please check the following thread https://communities.intel.com/thread/47989"
Windows 8.1: Very smooth ride but on the NUC it is very fast and may put you off other PCs.