3.0a11 is ALPHA software and does have some bugs. If you can stand it's little quirks it's great. I've been running it since it has been released.

Some of the features
Support for FLAC, OGG Vorbis
Crossfading
Gapless MP3 playback.

Those are the three that I remember most, I'm sure there are tons more. It is pretty stable, but you have to be prepared in case things do go bad. If you are unsure stick with 2.0 final.