I'm sick! So this was going to be longer but it will be quick! and also Quik was better as Quik rather than NesQuik. Just saying.
Every team has to worry about injuries but the Nats have a team with an especially robust injury history.
What starters missed at least 30 games last year? Zimm, Murphy (well...), Espinosa, Rendon, Werth
What starters missed at least 30 games 2 yrs ago? Ramos, Zimm, Espy, Bryce
Basically only Ben Revere has avoided major injury the past two season (we can quibble about Murphy if you like - he spent June 5th to the 30th on the DL).
Pitching wise it's a bit better with a lot of recent past season health but two starters (Strasburg and Ross) and the closer as currently defined by team Papelbon, have been injured this year.
Someone is going down. I'd bet on it. One, though, isn't going to derail the Nats. If three or four go down though? That's season changing and it's something that isn't unreasonable to think as a threat for this Nats team
2) Offensive decline
Even if the Nats remain healthy there are some reasons to believe the offense could drift off in the second half.
In Ramos, Espinosa, and Murphy, the Nats have three players having career years. These happen but when you see a surprisingly strong first half, it's likely that a more typical second half will follow rather than a repeat of what was just seen. This doesn't mean bad 2nd halfs (though Ramos and Espinosa certainly have that capability) but if all three slow down to paces more in line with their histories the offense can't help but suffer
In Werth, Rendon, and Zimm, the Nats have three players coming off of big injuries. Sometimes, if you haven't played an full season recently and are carrying lingering issues, the grind of a season can wear down on you harder. This probably won't be the case for Rendon. He had a strong 2014, is young, and has shown some spark so far this year. But for Zimm? Who last played a full season in 2013? Who looks so bad now at 31? I'm not hopeful.
In Werth the Nats have a player in his late 30s. Time catches up to everyone and nicks and dings wear harder and longer if your career is lucky enough to see the other side of 35. If it were one (injury recovery) or the other (age) I might give Werth the benefit of the doubt, but it'll be hard to shake off both of these.
Of course a moderate decline here - say a slowing of Ramos, Werth amd Espinosa, could all be washed out by Bryce turning into BRYCE again. And Robinson might play more if Zimm can't hit. I have a hard time seeing a completely offensive collapse. But a mild decline is possible.
3) Relief pitching woes.
Ollie Perez has disappointed. Felipe Rivero may have gotten used up in the first half. Blake Treinen's ERA hides a merely passable first half. It continues to feel like Papelbon gives up 2 hard hit balls for every one strike out. Solis has been unhittable, but only when he gets the ball over the plate.
The Nats don't have a terrible pen, but it feels a bit piecemeal when everyone is healthy. That's not where you want it to be 90 games into the year. Dusty has been pretty good at working around having a lack of go-to guys, but as we just saw, one injury, in this case to Papelbon, creates a chain reaction that throws it all into turmoil.
It could still work out well. 70 games is a lot of time and September call-ups may ID a live arm at the right time. But it could also fall apart. With an injury or two (God protect Shawn Kelley's arm) the Nats could be in real trouble. This is why when you hear about the Nats and possible trade targets you hear guys like Miller and Chapman bandied about. They would help set the 7-8-9 up in the traditional way preffered by baseball guys.
And the rest
5) Roark back and forth
6) Bryce merely being Bryce
7) Can Nats defensive holes (right side of INF, Werth) be exploited.
If you want me to guess how the second half goes I'd say the Nats get an injury or two, but nothing big. I think Bryce keeps up how he's been doing lately (not quite BRYCE but very good) but the offense still droops a little. I think the relief pitching woes don't get cleared up as the Nats make a deal - but for a middle relief arm that costs them little in return. In other words, I'm not really worried by injuries, and I'm even less worries by offensive decline. I think relief pitching worries are more on my mind but also least effectual, at least in the regular season. All in all they slow to a high 80s win pace for the rest of the games but that means 93-94 wins, and I think that means the East title.
We'll see. The big thing will be who gets injured - how bad for how long. It probably won't be enough as guys like Turner, Robinson, and Giolito are likely passable fill-ins in 2016. But there are combinations (Ramos & Bryce, Strasburg & any starter not Gio, Kelley & any other arm) that could make the road bumpier. Other than that though I don't see big worries.