the 770 is not abandoned. there probably be more official updates from nokia, and there will definatly be more apps.

there have been third party kernel for the 770 with extra features (large and fast mmc support etc)

i think the following bits are closed source in the software
power management
dsp (digital signal processing, ie the codecs for media)
opera (the is a mozilla based browser available minimo)
flash (gnash might one day be upto the job)
gtalk (there are open source options)

maybe someone can add more to this list.