We haven't been doing a great job of keeping this up-to-date, but it does still hit the major highlights.
Managed and Unsafe)NoEditAndContinue)FloatFast and FloatStrict)NativeWChar)OptimizeSize and OptimizeSpeed)NoRTTI)EnableSSE and EnableSSE2)ExtraWarnings and NoWarnings)require (modified)
debug.prompt (new)
http.download (new)
http.get (new)
os.chmod (new)
os.islink (new)
os.realpath (new)
os.uuid (can now generated deterministic name-based UUIDs)
path.getabsolute (new "relative to" argument)
string.hash (new)