In 2012 the Washington Nationals had only two players appear in at least 150 games (Danny Espinosa and Adam LaRoche) and only three with over 600 plate appearances (Danny Espinosa, Adam LaRoche, and Ryan Zimmerman). Compare that to the Nationals chief rival the Braves who had four players appear in at least 150 games (Michael Bourn, Martin Prado, Jason Heyward, and Dan Uggla) and five with at least 600 plate appearances (Michael Bourn, Martin Prado, Jason Heyward, Dan Uggla, and Freddie Freeman). And it becomes obvious very quickly that by simply having players play more often the Nats will improve.
Look at Jayson Werth who in 2011 had one of the worst years of his career batting .232/.330/.389 but played in 150 games and was worth 2.5 fWAR. Contrast that with 2012 were Werth rebounded in rate stats with a batting line of .300/.387/.440, but only played in 81 games and was worth only 1.0 fWAR. That is a win and a half difference for a player who in one season was a major disappointment and in the following rebounded significantly. The difference is two fold. There is the obvious playing time issue, but in 2012 Werth was rated as a well below average defender and some of that may have been due to the wrist injury. That is unknown, but what is known is even at Werth's well below average offensive year of 2011 he was more valuable to the team than he was in 2012. While a repeat of 2011's batting line isn't the desired outcome more playing time for Werth even at a diminished level will help the Nats. If Werth is even a 3.0 fWAR player that is still two wins more than he gave in 2012.
Take into account that Werth's injury in 2012 allowed Nady into the line-up and he was worth -0.9 fWAR as a National then the Nats got almost no production out of right field in 2012. Anything Werth adds in 2013 is going to be a bonus and he isn't alone as a player who by simply playing more will help the team. The next most obvious one is Stephen Strasburg who was shutdown near the end of the 2012 season. Strasburg's season stats are good and he was worth 4.3 fWAR to the team, but if he had continued to pitch at that level and had pitched 200 innings then he would have been worth 5.4 fWAR. Strasburg did have some control issues in his final few starts and did look like he was wearing down. If he happens to pitch at his career level for 200 innings then he could be worth as much as 6.4 fWAR. So we are looking at a one to two win improvement from Strasburg not being shutdown.
The next player that should see a significant boost in playing time wasn't even on the Nats in 2012. Denard Span hasn't played in over 150 games since 2010 and that was the only season in which he did so. In 2011 his career took a detour as he suffered a concussion and 2012 was his return to full time play. Even with only appearing in 128 games Span had 568 plate appearances or 4.44 per game. If he can play in 150 games he could get as many as 666 plate appearances and if he can do that at his career batting level of .284/.357/.389 then that will give the Nats the lead-off hitter they have never had and long coveted. Even if Span doesn't play that many games he will be replacing Michael Morse on the roster who was only worth 0.3 fWAR in 102 games while Span was worth 3.9 in 128 games. Even if Span doesn't play in 150 games he has a chance to improve the team by at least three wins simply due to his presence.
Relief pitchers and WAR go together like milk and ammonia. Drew Storen is rated as a 0.7 fWAR player in 2012 and a 0.9 fWAR player in 2011, but I doubt anyone would argue that Storen's 37 games in 2012 were just as valuable as his 73 in 2011. Throw WAR out the window since it isn't all that useful for relief pitchers and use common sense here for a second. Having Storen in the bullpen over Sean Burnett is going to be a good thing and it has nothing to do with Sean Burnett being a bad pitcher. Storen is simply better. With Storen though Burnett still would have pitched in 2012, but he wouldn't have been the primary set-up man. The absence of Storen is what led to the Nats trying Brad Lidge and Henry Rodriguez as closer and no one is going to make the argument that Storen isn't going to be a desired upgrade from those two. It is hard to quantify in wins exactly what a full season of Storen is going to mean in 2013, but it is going to mean a lot.
Last up are a bunch of guys that missed a little bit of time, but not a whole lot. Guys like Ian Desmond and Bryce Harper. The likely case is that Desmond slips a little in 2013 while who knows exactly what Harper will do. Many will say he is due for a sophomore slump, but Harper is different. That was on display in 2012 as he was one of the best 19 year olds to ever play the game of baseball, and if that trend continues and he is one of the best 20 year olds then Harper is going to not just add wins by playing more but by having one of the greatest seasons in baseball history. At the age of 20 Mike Trout had a 10.0 fWAR season and while that isn't likely to be repeated by Harper a season like Albert Pujols' 7.7 fWAR season at the age of 21 is well within Harper's reach. Even if Harper doesn't improve at all but plays in 150 games then he would be a 5.3 fWAR player. For a team in as tight a division race as the Nationals appear to be in for that 0.4 win improvement is nothing to take lightly.
Playing time is an underrated but simple concept. Teams are better when their best players are on the field. A lot of Nats missed significant time in 2012 and if more of the roster is healthy in 2013 then the Nats stand a chance to be better. It is hard to look at a team that won 98 games and say they should be better, but that is the position the Nats are in. If they can stay healthy this team has a chance to not just be good, but to be historically great. Even with the most conservative estimates from the above paragraphs the Nats could be looking at a six to seven win improvement from health alone. Not everyone will be healthy and not everyone will play at 2012 levels, but this roster is talented and 2013 is going to be a very fun season with a chance to be something we will all remember witnessing.