> http://www.mplayer2.org/comparison.html
> No longer depends on embedded FFmpeg tree or internal FFmpeg symbols

MPlayer required an embedded copy of FFmpeg to compile. This caused a maintenance burden as changes in FFmpeg fairly often broke the compilation of MPlayer. While it could link against shared FFmpeg libraries it would still use some code from the embedded tree instead, and also depended on internal FFmpeg symbols that are not part of the public API, thus making any dynamic-linked binaries liable to break when FFmpeg libraries are updated. MPlayer2 does not depend on embedded FFmpeg library copies and uses FFmpeg only through its public API. This eases maintenance and makes dynamic-linked binaries safe.

> Easy to use multithreading support

It's easy to build MPlayer2 with FFmpeg-mt support, and threading will be used automatically without requiring any manual configuration. This gives a big performance increase on multicore machines.

У mplayer своя ветка ffmpeg в кодовой базе из-за того, что они родное API ниасилили. В mplayer2 его осилили и даже сделали поддержку ffmpeg-mt. Из-за последнего mplayer2 обычно распространяется статически слинкованным с ffmpeg-mt, так-как в дистрибутивах обычно лежат старые версии (если лежат вообще). Но собрать без него и пользоваться системной либой можно без проблем.