I see two schools of thought for the defensive resurgence of late:
1) Dick Lebeau's "coach 'em up" ability has elevated their play, or
2) The injuries to the staid veterans has given some younger guys the opportunity to get some reps and build confidence, despite the whole "it takes years to learn Dick Lebeau's defense" garbage.
I think it's a current weakness of this team at a coaching/management level that there is a hesitancy to trust youth and athleticism in favor of experienced veterans that have passed their peak physical performance. Obviously you don't just cut all your veterans and go with inexperienced youth, but I think we all recognize that there's a tendency for the Steelers (and not just under Tomlin, I think Cowher was guilty of it too) to stick with the veterans and let promising young talent go underutilized and walk in free agency (like Keenan Lewis).
No disrespect to guys like Keisel and Harrison, because they've clearly had a big impact on the success of this team despite their relatively advanced age, but it's also good to see that some of our younger guys like Heyward and Spence have been able to step up. And I love Troy, but I honestly haven't seen much from him this season to make me miss him when he's not on the field. Between Gay and McCain, I think they have more interceptions this season than Ike Taylor has had in his entire football career stretching back to his Peewee league days.