I don't catch the main intent of the project.
Could you briefly explain how to use it and how it works?
From what I gather, it uses existing emulators as modules for a main set of wrapper code. Like MAME and MESS do with their various drivers, except using existing emulation projects like SNES9X, BSNES, Dolphin, FCEU, FBA, and dozens of others:
http://www.libretro.com/?page_id=218What most emulators out there seem to do is offer great core emulation code, but everything else sucks. They have limited command line options for front ends, crap ini file management, bad input/control mechanisms, no advanced output options for low res RGB CRT monitors or neat GLSL features for high res LCD monitors, etc, etc. I know I cringe every time I fire up some of my favourite console emulators when I've been spoiled by the advanced options MAME gives me. Similarly, I hate composite TV-out options from regular PC video cards because they spit out an interlaced picture. I've got RGB->SVideo converters, and would love to use a console emulator that allowed me to generate real console modelines out of a video card (like AdvanceMAME/GroovyMAME do), push that through my converter and play games in low res progressive modes on my CRTs.
libretro takes all the really awesome bits from console emulators (their core emulation code), and wraps it up in a very cool common configuration library that seems to be modelled quite a bit on MAME. It offers a single, consistent way to configure input (joysticks, mice, lightguns, etc) and output (screens/monitors, effects, audio, and even capture to a video stream via ffmpeg).
Anyone can code for libretro - there's even a handful of original games that use the framework (i.e.: not emulators, but standalone titles).
RetroArch is just one implementation of a simple frontend for libretro. No doubt others could make their own, or just very simply use existing frontends like Wahcade to power RetroArch straight off the command line.
MESS obviously does a lot of this already, but what libretro does is combine a lot of existing emulation engines into one consistent and very awesome tool. MESS and MAME also stick to their firm commitment of accuracy above all else, which is great, but means things like offloading 3D to a GPU will never happen in those projects. libretro takes existing emulation engines for 3D consoles and arcade games that allow GPU offloading of 3D (less accurate, but often prettier and fast enough to play realitime on cheap hardware) and gives a MAME/MESS-like interface.
I don't consider it a MESS-replacement, but rather a supplement. Tinkering so far has shown it to be quite an awesome little package. I'm considering building a Linux "Steam Box" next year for the living room (on top of my existing MythTV setup), and RetroArch would be an awesome addition to that.