It is not clear who will take over the starting job in place of Hall, but it could very well be someone who is not currently on the roster. I'm fine with releasing Hall since it was clearly necessary from a cap standpoint and he isn't nearly productive enough to justify such an expense.
Hall played very well twice against the Cowboys last year and also did a fine job in the win over Tampa Bay. However, he had other games, such as the aforementioned loss to the Steelers, as well as wins over Minnesota and Baltimore, where Hall was arguably the worst player on the defense.
I think Hall is a very effective blitzer, but as a cover cornerback, which is where all cornerbacks must make their bones, he was normally mediocre or worse. He gave up 374 yards after the catch last season, third-worst among all NFL cornerbacks, according to Pro Football Focus. What's more, opposing quarterbacks completed 67% of passes on Hall, a terrible figure. That probably explains why he was targeted so frequently -- 112 times in 2012 -- tied for second-most in the entire league.
The Redskins don't have much at the cornerback position right now. With Hall gone, that leaves only fellow starter Josh Wilson, the best cover man on the team, to go with Richard Crawford, a 7th round draft pick in 2012 who saw most of his action on special teams. Cedric Griffin and Jerome Murphy are both free agents.