Dickens basically tells his critics to kiss his rear end. Which, honestly, is business as usual as far as his prefaces are concerned.
Dickens declares that he knows what he's talking about, and however viciously he mocks the government or the supermoguls of his time – well, if anything, it's not nearly viciously enough. (See "In a Nutshell" for some background on this.)
Oh, and also, the Marshalsea prison doesn't exist anymore.