Some general thoughts on what it takes to have a good secondary:
1. At least 2 really good players, with another couple who are solid. For Seattle, Sherman and Thomas are studs, Chanceller and Browning are solid. For Pittsburgh, Polamalu has been the stud. Clark and Taylor are solid and then some. For Baltimore, Reed has been the stud and Lardarius Webb was emerging as one. For San Francisco, Goldson has been a stud and the other 3 starters have been solid.
2. Continuity of personnel is key. Pittsburgh has had Polamalu, Clark and Taylor together forever. Seattle's young secondary has played together for 2 years now, as has San Francisco's. A constant revolving door of players learning the system doesn't work. Injuries are a given and take a toll, but the good secondaries are the ones with some stability and continuity.
3. Continuity of approach is key. Matching personnel to a scheme that fits them, and don't break it. All secondaries play different cover schemes, but there has to be a basic continuity. The Pats seem schizophrenic sometimes between man and zone. They played successful aggressive press-man against Houston, then started out in a soft zone against San Francisco. It's very confusing.
4. Coaching is obviously a factor, as we've seen certain teams be more successful than others at developing young DBs. Again, continuity of of scheme and personnel make it easier to add new players, then when everything is constantly changing.
5. Having a "QB" of the secondary - usually a safety - helps to facilitate communication.
I personally think that the Pats have a good nucleus for a talented secondary. McCourty has the ability to become an Earl Thomas kind of force at FS if he is allowed to stay there and develop his instincts. Talib isn't Richard Sherman, but he's Ike Taylor, or perhaps better. Alfonzo Dennard is very talented. That's a solid start to build from. I haven't discounted Dowling, and guys like Gregory, Wilson and Arrington are much more effective when they are playing defined roles as part of a clearly structured system than when they are being moved all over the place. Moving people around constantly disrupts continuity, creates confusion, puts players out of position relative to their skills, and generally hasn't worked well. I'm very alarmed by this approach, and think that it has ******ed the development of our secondary. JMHO.