There is no doubt that Williams is a very good defensive coordinator. Much of what he's done there in Washington has been smoke and mirrors. He hasn't always had talent to work with.
Like you say, Taylor has talent. Springs has some, although I think Williams' schemes have helped him out some. Rogers presumably has talent, although he didn't light it up last year. Griffin has talent too, and Washington.
However, 3/4th's of your D Line has been somewhat less than stellar throughout Williams' time in DC. Wynn, Evans, Daniels, etc. at DE just isn't very good. Although I don't think Carter is that good, he's a real step or two above who you've had there at DE... so he should help. Salave'a isn't real talented, but I think he's better than a lot of people give him credit for... at least in Williams' system. Still, he isn't a world beater. Overall, the line has been pedestrian except for Griffin, who is good, but not great.
The fact that Williams gets what he gets out of that group is a testement to his coaching ability and schemes. There's no doubt that he's good.
The LB's, while easy to plug and play in a 4-3, always seem to play good for Gregg too.
Without Williams the Commanders would've lost a lot more ballgames over the last few seasons than they did. You better be damn glad he didn't take a head coaching job somewhere.