What is missing from OpenJDK?
Sun's JDK 6 is based largely on the same code as OpenJDK 6. A lot of good work has been done to make OpenJDK a solid development and production platform. The primary missing feature is WebStart. The OpenJDK is perfect for running things like Tomcat, JBoss, Resin, Eclipse, etc.
On OSX there are a few known issues with OpenJDK6 that we've run into:
#1) It doesn't work with Eclipse (Ganymede or Galileo) without hacking. My understanding is that this has been fixed for OpenJDK6 and the newest version of Eclipse under development.
#2) The text and graphics in Swing Apps looks poor/funky.