2009-2010 iMacs with ATI 4xxx GPUs
Fixed in version 2.2. Finder quicklook plugin did not work. This ATI GPU generation was not suitable for high-performance OpenCL rendering. (ATI 5xxx and later generations are fine.)
Plugin no longer uses GPU for rendering.
2008 Model Dual CPU Mac Pro
Color gradient editor does not work. This turned out to NOT be a bug.
Resolved: This was due to a corrupted User Preference file. Deleting the user preference file for Fractal Architect fixed the problem.