As others have said, Disney Gift Cards are widely accepted within the parks themselves. Along with the balloon sellers and Ghiradelli*, the cash only walk-around vendors (likes the ones you sometimes see in the attraction queues) and coin press machines do not accept gift cards. I'm also not certain about the lockers...Guest Services said they accept gift cards, but gift cards were not listed on the accepted forms of payments on the rental machines themselves.
*Along the lines of the Ghiradelli, I would also wonder if the Arribas Brother shops on MS and NOS accept gift cards.
Outside of the parks, it's much more hit-or-miss. The Disney owned shops (e.g., WoD, Pin Traders, D-Street, Marcelines, etc.) in DTD accept gift cards, but the third party shops (e.g., RideMakerz, Build-A-Bear, etc.) do not.