As a San Diego International Comic Con virgin (who will sadly remain so until at least next year), I'm perhaps ill-equipped to articulate the importance of the Comic Con bags. So far as I can gather, they don't contain anything particularly cool, unless you decide to put a dinosaur in one of them.
They're bags. Plastic bags. With pictures on them.
Specifically, pictures related to TV shows Arrow, the DC Nation family, Supernatural, The Big Bang Theory, Fringe and The Vampire Diaries, and forthcoming feature films Man of Steel and Pacific Rim. The official designs for the TV bags were revealed today on the official Warner Bros Comic Con site, though they're keeping Man of Steel and Pacific Rim under wraps for now.
Apparently they're a big deal, and you can collect them. If anyone's passing that way, I wouldn't mind a bag with Jensen Ackles on it. I need somewhere to keep my dinosaurs anyway.