Press Play's crap.
It takes forever to start an MKV (Does some whack processing stuff at the start) , and the playback from the files I've used was abysmal (Whereas the trials of mobile.HDplayer/Power DVD were fine for playback)
EDIT : To expand on my point, the time it takes for PressPlay to do its half arsed conversion thing, I can do a quick decent quality MP4 encode which will play perfectly fine being streamed over the network while having a smaller file size.
Or I just use my X86 Laptop with desktop software and have it running perfectly fine.
I wish Baytrail was out, I was tempted by the Asus ME400C when I was in PC World the other day.