When someone instant-messages you, a pop-up appears over your panel.
You can join an IRC channel and chat with numerous people at once. If you're on the IRC server, but not in any channel, if they send you a private message, it's just like receiving an Instant Message on any of the other services! You can watch who comes online, too, just like IM!
If someone mentions your name (or any other interesting keyword) in an IRC window, you can have Kopete highlight it and even play a sound.
Hate having tons of IM windows on the screen?...
...stick multiple conversations together into one window using tabs!
Like most other KDE apps, you can specify exactly what Kopete should do when certain events take place. Pop-up a window? Play a sound? Log to a file?