If it's been great until the end of the keg, then it's got to be something going on with the lines/tap. I mean, the keg has been sitting for a long time, so yeast would not be an issue. But if most of the keg is gone, and the beer has an harshness now, I'd think that some of the flavors have melded (allowing any off-flavors to be more appearant) or the kegging gear itself is to blame.
Did this just start, as the keg cleared up?
Broken Leg Brewery
Giving beer a leg to stand on since 2006