Defense - particularly in the front 7. Some of that may be due to players, but Bronco appears to have made huge strides here.
A lot of good changes are really just fixing mistakes that never should have been made in the first place, though. For example:
1. Conditioning
2. Dropped passes (2010 was ridiculous)
IMO, we are also losing ground on a lot of things that we generally did well:
1. OL production
2. QB
3. Route-running
4. FB (especially in blocking - we miss Tonga dearly)
5. Kicking