Probably one of the nicest features so far is the download page. Select the components that you need and dependencies will automatically be calculated for you. You also have a choice between compressed and non-compressed versions for download.
That aside, some of the things they mention include an updated Class system that more closely relates to Dean Edwards' Base; Element methods automatically available through $('id'), something that is in the recent Prototype builds already; chainable methods allowing things like $('el').addClassName('foo').removeClassName('bar'), which is already in JQuery and has made its way into Prototype; and a new effects system, making it easier to add simple effects to elements.
The Moo team pride themselves on compactness and this library seems to do just that getting the full library compressed down to 20k. That's including features like Ajax and drag and drop.
With no demos and little documentation, it'll take a bit for mootools to gain any traction. Plus, with the popularity of JQuery and Prototype, and especially with recent changes to Prototype which introduced things like chainable methods, will mootools be able to make a name for itself?
Update: If you're interested, I took a little time to put together a drag and drop example using mootools.