Is bringing Wang in on a deal a good idea? Depends on the price of course. It's certainly a more promising deal than Estes, or Batista, and it wouldn't be a waste tossing out a low-ball offer to him. Then again let's not confuse Wang with a Sheets or a Harden type-project just because he was doing well before.
You see CM Wang spent his pitching life walking a very thin line. He was a groundball pitcher with very good control. That great control was able to make up for the fact that his "out pitch" was not a pitch batters swung and miss, but instead hit. He kept extra men off the basepaths by not issuing walks, and he kept the other team from getting the big hit by keeping the ball down in the strike zone. In numbers from 2006-2008
K/9 BB/9 HR/9
2006 3.1 2.1 0.5
2007 4.7 2.7 0.4
2008 5.1 3.3 0.4
While, as Chris said, you would really prefer groundball guys that strike people out, this method can work. Of course any deviation from it and the delicate balance can be ruined. Last season Wang lost his control (4.1 BB/9) and got hit much harder (1.5 HR/9), despite the fact he was getting more K/9 (6.2). Add in a little bad luck with a BABIP around .400 and you go from long-term star to out of the league is just 40 innings. The difference really can be that slim.
Wang almost certainly is better than the hide your eyes effort he put out last year. You have to figure those groundball are going to find more gloves. But if the walks don't come down, if the pitches don't come down, he's not going to be useful. This is what the Nats need to be sure of before taking a chance on Wang. If he looks like he's hitting his spots, don't worry about the velocity so much. That'll come back with time. But if he's not, well... the deal should be a lot lower.