Jason Heyward and Jordan Walden for Shelby Miller and a prospect, Tyrell Jenkins.
Short of it : Heyward was a very good OF, but was up for FA after next year. Miller was once seen to be a definite ace, now a useful rotation piece with potential. Both very young. Other players dealt could be see as trading the now (Walden) for the later (Jenkins)
Long of it : Given the Braves aggressively resigned several players to long term deals so it's likely they tried the same with Heyward. But they were offering under market deals, trading years for dollars. Signing that kind of deal made sense for guys with a lot of control years left, but Heyward had gotten too close to FA for that to work effectively. He signed only a 2 year deal to cover arbitration years (sound familiar? That's what Desmond, Fister and ZNN all did) looking to score big as a young guy (will turn 26 in August) hitting FA. How big? I don't know. Choo signed for 7/130, he's better than that so... 7/160?
The Braves knowing that they wouldn't pay for that decided to deal him. Why wouldn't they pay for that? Well, Jason had been slipping at the plate, his power disappearing to the point where his value was tied mostly to his average and a decent walk rate. It's possibly they could be paying 25 mill a year for a .260 singles hitter come 2018. But his value isn't solely in offense. Jason is one of the best fielding corner OFs in baseball. While last year's WAR might be in part because of the vagaries of annual defensive stats, no one doubts he's excellent and will provide value there year in and out. But is it enough value? The Braves thought not (an injury history also didn't help)
The Cardinals found themselves in need of an OF after they dealt away Craig and Oscar Taveras died tragically. They have some depth in starting pitching with Wainwright, Carpenter, Wacha and Lynn being under control or contract for several years. They also have Carlos Martinez, a very highly thought of prospect, waiting his turn. Miller was deemed expendable. He was formerly thought to be an ace in the making, but over the last few years had slightly disappointed in both the minors and the majors after fast starts. Last year was up and down enough to give the Cardinals doubts. Walk rate up, K rate down, but a September that brought back the idea that he could front a rotation. Would he put it together? Or would he flounder? There isn't any doubt he's rotation worthy but is he a 1/2 or a 3/4 remains the question.
The Braves were looking for someone to control, and an arm would be nice given their issues with starting rotation health that helped bury the team in 2014. The match was made and here we are.
Who won: On value it's hard to argue anything other than the Braves won. They got a rotation pitcher on the cheap for the next 5 seasons. The Cardinals got a guy who is perhaps a 1 year rental.
But a look any other way favors the Cardinals. For 2015 specifically, Heyward shores up a place of need without immediately breaking the bank, and Walden helps as well. Going from a good OF defense to an very strong one, without much offensive compromise the Cards may have made themselves favorites in the NL. The Braves on the other hand had terrible offensive struggles and just dealt one of the few guys that was not an issue. As constructed today, you'd expect a season full of 3-1, 4-0 losses.
Also the Cards dealt from a strength and shored up a weakness. They have enough arms to cover 2015 and some intriguing ones in the minors. The Braves dealt from a weak position (after the fair prospect Terdoslavich, the Braves minor league OF is barren) to add to what could be a crowded position. Teheran, Wood, Minor, Medlen, Beachy have all been rotation worthy over the past 2 years and they also have a couple decent minor league arms. There's no such thing as too much pitching, but there is such thing as too little outfielding.
What does this mean for the Nats? : We'll have to see what the Braves do right now, but as of November 17th the Nats are looking at an easy road to the pennant. Miller should be at least ok, but unless he's Kershaw GREAT it's not going to make up for the loss of Heyward. They had plenty of decent 2014 pitching performances. Right now I'd have trouble pegging the Braves at anything over .500. That leaves the door open for the Mets or Marlins. Right now the Marlins still need starters, but like what they have developing and are likely to wait a year or two before going for it. The Mets still need bats. They aren't as bad off as Mets fans would have you believe but they do need to score more to be a threat to the Nationals. SS is their biggest hole so Tulo is bandied about but mostly in jest. Having signed Cuddyer the Mets could make a play at another QO turning down FA. Do they go after Hanley Ramirez? or David Robertson as the pen needs help, too? or something crazier like Scherzer trying to go toe to toe rotation wise? Or given it's the Mets do they do nothing and overpay for Asdrubal and finish 83-79?