We already looked at the offensive side of things here and now it's the defense.
Here are the three changes from 2009's first depth chart to 2010's first depth chart:
ILBs Demorrio Williams/Corey Mays (Derrick Johnson/Jovan Belcher): I feel that this is an upgrade overall. DJ seemed to make plays when he was in the game last year and Williams didn't have a sack or an interception last year so I think DJ gives the Chiefs a better shot of making big plays.
I'm not sure that the defense got a whole lot better not counting development of guys already in place, other than safety. Edwards and DJ/Belcher could be considered upgrades but not big enough that I think it's a significant difference.