When you start the launcher, log in using your username, it downloads all content but you just end up with black screen, using the MinecraftDev launcher fixes the problem.

Also, regarding the system, I also tried it on Ubuntu 12.10, not only Kubuntu 13.04, don't take the "Affected Version" as a valueable thing, I just selected it cause that value is needed, happens on all versions if using the old Minecraft launcher