Re: Why did the League of Shadows want to destroy a peaceful city?
I think the answer to the question of why the League would want to destroy a peaceful city is simple. Bane refers to the peace as "borrowed time." Time, borrowed by Batman's victory over the mob. I doubt Bane and Talia were rooting for Gotham or even paying attention to it's progress in those 8 years.
The problem with Batman and Gordon's cover-up, is that it's just that, a cover up, bandage solution that did not get rid of the core problems in the city. The movie suggests numerous new problems as mentioned by other posters. Unemployment, white collar greed and corruption evidenced by Daggett, and laziness, decadence, and lack of vigilance on the part of the the GCPD, illustrated by Foley and the Senator at the beginning of the film.
I think a lot of people are underplaying the significance of Selina's criminal background as something she saw as a necessity. Her resentment toward the rich and her initial sympathies toward Bane's cause is another indication that all's not right in Gotham.