On KDE, the "new message" tray icon is the same as KMail's
When using Quassel on KDE's plasma desktop along with KMail, and both are in the systray, Quassel shows the same envelope icon as KMail uses when a message with the user name in it arrives with Quassel not being in the foreground.
While this is of course not a bug, it's not nice. So maybe, you want to use a (slightly) different icon for that situation? Something like a speech bubble or such?!
However thanks for your this great piece of software that makes using IRC so much more fun!