I solved it!
http://wdc.custhelp.com/app/answers/detail/search/1/a_id/5385/c/130/p/208,393
Apparently, the Samba network protocol, which is used by WDTV for network sharing, does not support high bitrate video streaming. The workaround is using a media server program, then accessing full HD videos through Media Server option on the WDTV. I would recommend installing PS3 media server, it’s free and IMO it is great, it understands WDTV automatically without any problem, and also the video reproduction is very smooth, and the libraries look quite decent.
