The ability to slip audio and picture at the same time as to keep clips in sync with audio.
The ability to use JKL as to actually play video and or audio while slipping as opposed to just dragging.
The ability to use JKL as to scrub your audio while slipping so you can actually hear what you are slipping and have more precision when slipping dialogue.
The ability to slip in a four up display as opposed to just a two up display so you can actually see what it is you are slipping against.
The ability to choose an a or b priority side to appear in the client monitor so you can actually see what you are slipping.
Usually when people slip they have an A or B side priority, they need to keep things in sync, they need to see what they are slipping, they need to hear what hey are slipping and they need to see the other shots that they are slipping against.