IMHO, the defense was poor because:
Green and Thomas were effectively dead weight.
Burgess didn't start "getting it" until way too late.
We had one safety, Meriweather, who's very good in coverage but can't tackle worth a damn (Bodden was better), and another safety, McGowan, who's a stellar tackler but who screwed up coverages so terribly that BB had to bench him for Sanders (same problem McGowan had in Chicago, IIRC). Getting a pair of safeties who can BOTH cover like Meriweather AND tackle like McGowan might make as big an improvement as getting a hotshot pas-rusher would.
Maybe Chung will replace McGowan in 2010. If BB doesn't think so and if Lockett and/or Arrington don't appear to have enough potential, we might see BB take another safety in this year's draft.