Hi,
Lets see. In no particular order...
680x0 development has stopped but the core is still going as coldfire, which at least one guy here is determined to put into just about every 68K amiga model going. Given that the coldfire runs up to 300+ MHz, I hope he succeds :-)
OS4, OTOH, is PowerPC based (60x/G3/G4). It is targetted to run on the AmigaOne series and also (at the time of posting) the CyberStormPPC accelerator for the A4000/A3000. Emulation of the 680x0 processor is built in to allow older 68k programs to function. The emulation is based on a JIT technology rather than interpretation.
The main language for development is still C/C++. The developer tool of choice IIRC is gcc ;-)
OS4 should be largely compatible with 3.1. Given the amount of time and effort put into it I wouldn't hold your breath for an open source version. If that's more your bang, however, check out AROS.
Supporting other filesystems has been there since v2 IIRC
