Tactical MarkersThe overlay lets you place markers at your character's location in 3D in the game world. You can then see the location of these markers from anywhere on the map. The icons of the markers are customizable (although you need to edit the xml files by hand for now), and they can disappear until the daily reset (the included Orphan map is using this feature).
Map TimerA configurable map timer is included in the first release, by default it is set to display the HoT map rotation.
Mouse HighlightThe third feature which I found very useful in raids is the full-screen mouse crosshair.
All of these features can be configured and turned on-off.