...A great deal can be done on and with them ...still...
there are a couple of pdf viewers that work fine.
skype may be obsolete ...but I still use it on my n8x0's...works fine for me. Not sure if gtalk is still working ..haven't used it in a yr-ish now...but it was working back then.
...they are pretty darn cheap now nokiabot...grab one and mess with it.
I think they are worth the effort.
So far...these guys think so too..see what the future brings...

April 2nd 2015-
"The Nokia N810 and N800 tablets will going to kernel developers that are working on Nokia's old Linux devices and upstreaming drivers"
http://planet.freedesktop.org/

As far as what doesn't work anymore,
or properly anymore,
or does work well or middling,
or even possibly "could" work...
depends on whether you are still running stock diablo...
much of that is or may be dependent on what you have done to "beef-up" your diablo or your alternate os you are running or the chroot you run and what programs you then (may or can) utilize.

I'm not saying that the devices are a limitless well of possibilities ...able to do it all...but there most certainly is more that can still be accomplished with them... than many think.