The move from a strong offense to a weaker one made a lot of people drop Werth lower in their outfielder rankings. The idea is he will score and drive in less runs with a weaker supporting cast. While it is true he will likely hit with less runners on base the Nationals made an announcement that may lead to an increase in runs. Jayson Werth will be hitting in the #2 spot in the lineup meaning he will have Ryan Zimmerman and Adam Laroche hitting behind him. It's a pretty big improvement from Raul Ibanez and Carlos Ruiz. The spot in the batting order could also add about 30 plate appearances over the season meaning a few extra HR and SB chances.
The Nationals will rely on Werth to get the offensive game going and they may get him to run more often. He has an excellent 53/7 SB/CS ratio over the past 3 seasons so he should be running more than the Phillies had him going last year. The Nationals will need to produce runs any way they can and it wouldn't be a surprise to see Werth approach 25 or even 30 SB.
With run producers hitting behind hin he should score as many runs as last year if not more and should add a few SB from a year ago. RBI chances will drop but the increase in plate appearances should make up for some of the loss. He still has above average power so the move to a less HR friendly park shouldn't hurt too much. Werth should be able to make a run at a 25-25 season and remain among the game's top outfield options.