"Does MKV now support forced subtitles?"
I can say that by default on Plex, forced subtitles are not supported. I haven't had the time with it enough to determine the work around. No other negatives at this point. I imagine any other negatives I find will be far outweighed by the benefits. So far Plex offers the same practical functionality as my previous mediaserver/frontends, but I am not too happy with the GUI (and simple end-user customization) or their lack of a formal skin gallery. The ability to effectively stream to my bluray mkvs online to a webbrowser (and look better than Netflix!) or my android tablets/phones and Win PCs/laptops, puts it over the top for WMC (my current frontend) and my old beloved Xlobby. Fortunately, I don't use my HTPC for managing my DVR/live TV, so I don't miss not having these features on Plex.