Someone like, say, Michael Chrighton, or Freeman Dyson, is vilified for speaking out against AGW, especially given their lack of expertise in climatology. Al Gore, however, is treated like a hero, despite having not only no experience in climatology, but his total lack of scientific expertise, because he espouses an opinion for which there is scientific consensus.

Uh, yeah?

So Al Gore is acting (to a degree) as a spokesperson for the scientific consensus.
(Consensus based on repeatable results, not just "we all agree with that guy", by the way.)

Anyone taking a contrary position to the scientific consensus will need to have some fairly impressive scientific results backing them up, if they want to claim the majority of climatologists working in their own specialist field have got it all wrong. If you haven't got some real compelling research, and it's not even your field of expertise, then you run the risk of looking like a crank.