No it will not be annoying. 43dB is at load (which it won't be under XBMC). You would barely hear it from 3m away (it would be a very soft whisper, or equivalent to the background noise in a recording studio). You might just hear it if you only use XBMC to watch silent movies in a totally quiet house, in the countryside.
I have an i5 in my laptop, and while using Chrome, Outlook, other programs, and running XBMC at the same time in a window, it uses no more than 15% CPU.
So lets start with the 39dB figure 12.5cm away from the box which is the figure stated in the review.
Calculation
Source for more information on PC noise:
Archive: A Primer on Noise in Computing | silentpcreview.com
From page 2 we learn that volume reduces by 3dB each time you double distance. Applying this concept:
39dB @ 12.5cm, 36dB @ 25cm, 33dB @ 50cm, 30dB @ 1m, 27dB @ 2m, 24dB @ 4m.
At between 2-4m away from the device, the loudness is 24-27dB = background noise in a recording studio or a very soft whisper.
Conclusion
The new NUC is not noisy, the airflow from the fan does not affect viewing pleasure, and this completely matches the experience of many others on this forum who use Celeron based devices with a fan who state it is quiet. There is no noise problem.
Edit for Solamnic - in another thread you say that the Scythe Big Shuriken 2 is totally silent. This NUC is the same loudness as one of these fans running at full speed (30dB @ 1m). Source:
http://www.silentpcreview.com/article1270-page6.html.