Special in-game features (different background of the main screen, etc.)
I completely disagree. For one thing, in-game stuff remains "collectors only" for a couple days, tops. If there is a code for it, someone spreads it around the internet, or someone creates a very simple mod that unlocks it, etc, etc. There are tons of ways to make it available to everyone, at which point it ceases to be "special."
If the special feature is meaningful, then it is unfair to people who just buy the regular edition. If it isn't meaningful, then it wasn't worth the extra money to get it.
I think the collectors edition should include the soundtrack CD, Frogboy's book (please?
) mycolors theme, and maybe little pewter figureine. Kind of like the Rook figure that came with the Demigod Collecters Edition. Maybe a dragon or something.