First post, yay!
To the topic, being a recent S4 owner and completing my build. I was eyeing the R9 nano since it's come down in price in Europe quite nicely.
I stumbled upon countless threads about coil whine, but the lack of HDMI 2.0 port was the final deathblow for me. I can't imagine why they crippled a HTPC / mini-itx system oriented very powerfull card like that. I really hope future brings us more cards like the nano but with a better design / components.
Lack of HDMI 2.0 means no 4K @ 60hz, and there are literally no high res televsions with a displayport connector on them.
I know most ppl don't use their TV's as monitors so it's not a problem. I have a 30" HP monitor for serious gaming, but I also enjoy hooking up the mini to my downstairs 65" 4K tv, grabbing a controller and playing. Much more laid back and I really enjoy it.