Sorry for bumping an old thread, but I just got a new computer (right now using the AMD graphics card built in) and I started having this problem last night and it continues into today. It started in Gridania Hamlet Defense. Zoning fixed the issue. Same thing happened in Ul'dah so I ran into Zone B and characters displayed again.

Seems like the issue is heavy loads cause nothing to render. Won't be able to test it on my Nvidia card until I get a new PSU, but hopefully that will fix the issue...