I voted rotation. If the question is asking about the biggest season-long concern going forward, it's probably going to be the rotation. We're probably in the bottom 5-10 in baseball there. A 4ish ERA from Lannan, Marquis, and Gorzelanny is probably the best we can expect. Who knows what Livo will do. And Zimmermann is kind of a wild card for the 150 innings or so that he'll get.
If the question is asking about the biggest concern so far, it's probably the offense not being able to generate enough hits. They're getting on base at an OK clip due ot the walks, but they need to drive those guys in at some point. Morse and LaRoche are so ice cold right now. But over the course of the season, the offense will work itself out and we'll be in the middle of the pack there. The rotation isn't going to get any more dependable unless we bring in new personnel.
The defense - they've brought in a bunch of players who are supposed to be solid defensively. I don't know why they've begun making mistakes now that they're in Nationals uniforms. Sucks for us, but I don't htink it will continue.
The pen is a relatively easy fix. Broderick needs to be in AAA one way or another. If Gaudin doesn't cut it, then cut him lose. Pitchers like that are fairly easily replaceable. Same goes for Coffey. And they have several right handed relievers who are very tough on lefties, so that may make a Slaten optioning less painful if it comes to that.