Engadget says:
"Fortunately, custom tones made the list of 500 features, but there's a twist: you can only get these custom tones by loading an MP3 or WMA clip no longer than 40 seconds into Zune, placing it into one of your music folders, right-clicking the tune in order to edit it and changing the song's Genre to "ringtone." Then simply sync to the phone, select the song in your device's settings and viola! Nothing to it, right? Easy peasy, if you have software that can crop your favorite music into 40-second segments. Otherwise, get used to the awesomely bad MIDI-style beats found in your phone's ringtone catalog. Don't get us wrong -- we're glad to have the opportunity to throw in our own songs now, but there has to be an easier way."

Although Microsoft says there are 500 new changes in Mango, I don't think there are many visual changes other than the multi-tasking feature.