More channels is the biggest...but what I really liked about it was the functionality of the digital cable box. The cable company that I used at least, had a TV Guide thingy that they piped through the box that you could use to check out what was on for the day...and what was going to be on three weeks down the road.
You could look up a show and then see when it would be on again. Pretty much any info that was in the paperback version of TV guide was piped over an accessable on my digital cable box...minus the articles. It was very handy!
Outside of the more channels, and the tv guide though, it actually had worse picture quality IMHO. I had digital artifacts showing up regularly that would pixelate the picture. They would last anywhere from half a second, to half a minute :|
Cable company said they couldn't do anything about it.