There's a complete change log online if you want to get into the nitty gritty of what's included, but the biggest additions include multi-touch gestures, Growl notifications, support for Apple's Cocoa application programming interface, and restored support for Macs running the old Mac OS 10.4 Tiger operating system.
There are still many older Macs out there running Tiger, so it's good to have that back; just be warned that you'll still need an Intel processor to run Opera. The older PowerPC processors aren't supported no matter what operating system you're running.
Now that Opera uses the extremely popular Growl notifications software, it can show quick, transparent updates on your screen when something important happens. Gestures for multi-touch screens sound like a sweet idea, but we haven't had the chance to try them out to see if they're actually useful — let us know what you think in the comments if you give them a spin.