My favourite part of unraid will definatly have to be either the community or how easy it is to learn and use. Unraid has taught me so much in the past 9 or so months and has driven me to experiment more and more with computers and OS's thanks to VM's.
If i had to ask for one thing it would have too be some raid options for cache. And maybe even the array. Although i know that kinda defeats the "Unraid" part.