I'm a Linux user, so this is my disclaimer: I don't have any problems right now. However, a LOT of Mac OSX users have reported the following problem:
"The application just bounces in the tray for a bit, and the game doesn't start. If I use the web client, it hangs after login."
This *MAY* be a problem with the Mac version of Java for 32-bit systems. What does that mean? It means that your OS may be fighting with Java because of a new java version.
I need someone who is having this problem to try the following: