With the re-signing of DeAngelo Hall yesterday and today's acquisition of Albert Haynesworth, the Redskins defense looks pretty solid on paper at least. And consider that last season, without Haynesworth, Hall for less than half a season and Jason Taylor banged up, the team finished sixth in points allowed and fourth in yards allowed.
Of course, Taylor could retire or get cut, (though that appears unlikely) and even if an entire defense comes back intact, that doesn't mean it'll repeat the previous season's performance. But Haynesworth is a major difference maker up the middle and has the ability to collapse the pocket. If Taylor returns close to his previous form (defensive ends tend to age well) and Andre Carter also contributes, this could be one of the toughest defenses in the league.
Just keep in mind that Dan Snyder has a history of vastly overpaying for past-their-prime players and coaches (Taylor, Mark Brunell, Bruce Smith, Adam Archuleta, Deion Sanders, etc.), so it's hard not to assume something will go wrong.