The main thing I'm looking at right now (circa October 2010) is PDF rendering on Linux, which primarily means the poppler library. Epdfview and Evince are document viewers which use poppler and gtk+ as a backend, Okular is a document viewer which uses poppler and Qt as a backend.
I have had trouble with viewing some PDFs, and there have been a lot of poppler-related bug on Ubuntu's bug tracker, so this is what interested me.
One problem I have had is displaying my local bus map with Evince (using the poppler library) on Ubuntu 10.10. It takes 16 seconds for the map to come up. Eight seconds of that is poppler rendering, the other eight are evince. I wrote a blog post about this. A commit from November 2009 to allow better text selection from tables, by doing a topological sort of the PDF, is what is doing this. Ideally, we would have that fix for selection in place, but also speed up display for PDFs such as this map.
Evince is breaking on Ubuntu 11.04 alpha on gobject introspection suff. But it is very alpha right now so....
Evince or poppler is not printing filled in forms correctly.
As cairo has not implemented stroke_to_path() yet, some PDFs will not display. Poppler has been following this since at least August of 2009.