re: Explain to me why the Saints defense will be better this year.

A new system and new DC which both cater to getting pressure on the quarterback (something that rarely occurred last year), a new corner that actually gives us 2 viable starting corners, a first round draft pick that hopefully can be an immediate contributor and can actually cover the slot. Also a system that allows players like Hicks and Galette who were great in the limited playing time to see a lot more action, and creating a suitable position for a tweener like Martez Wilson that naturally suits his size and speed.....

ETA: Getting rid of Shanle and Ellis = addition by subtraction.

As far as the 2nd part of your question, our offense will take care of the rest. Our defense just needs to go from Worst to average for us to compete with the teams you named.