i believe native support for flac streaming is not available in WHS2011 (amazingly?!) so if you favour quality audio then avoid (although with some arseing around its possible)
also windwos software raid is not too good and ive read a few bad reports about whs for when it fails and performance related woes too. Also typically windows doesn't play well streaming to linux units and performance is weak there too which might become apparent in every day if you use wireless and of course if you end up running xbmc as an o/s via openelec or xbmcbuntu then you could run into problems.
for home storage and stream id try freenas 7, or 8 if your going for a big setup and dont mind putting the time in. Amahi looks good too as mentioned. alternatively tvmobli is easy to use and works well, it even allows for playback of flac on ps3 as it realtime transcodes your collections to formats that then end device supports, it works on mac,pc or linux