Oh, and I keep getting "Video could not be decoded" when using stock youtube player.. I am loving the hardware of the nokia, but I am starting to relate it back to the old symbian phones..(Not good)
My note 3 worked flawlessly on everyting, except they removed sdcard writing on kitkat.. I want the 1520 to be good, but it is hard to see right now..
The "stock YouTube player" is just as wrapper for the mobile YouTube page, as we don't have an "official" YouTube app.
Try MetroTube or MyTube. These are the best 3rd party YouTube apps available and are quite good.
As for your NAS, maybe the Remote Desktop app will help if none of the apps mentioned above do.
For mkv files, Moliplayer pro plays them yes, but I prefer PLEX streaming media server. Download the free PLEX media server software on your PC, create an account, and set it up by telling it which folders have all your movies/videos. Then download the PLEX client app (was $4.99 when I bought it - the best $4.99 I've ever spent!) Fire up the app, sign into your PLEX account, and enjoy all your movies streamed right to your device, no matter the format mkv or whatever.. One of the best and most used apps on my Surface 2 and my 1520...