Not so much a "mistake," but if I had our first visit to do over again, I wouldn't have invested in the Disney dining plan (standard). The four of us couldn't remotely eat enough to make it worth our while, and having to make so many table-service ADRs limited our touring flexibility. (Now that both of my kids are over 9 and would have to pay adult prices, the DDP would be even worse for us.)
*Caveat: On a couple of subsequent visits when the DDP was offered "free" (at a discount), we were able to use it very successfully, but I'd never pay full price for it again.
Honestly, other than that, between the Unofficial Guide and this forum, we got so much great advice before our trip that we didn't have any other "noob" mistakes or missteps! Unless you count stepping foot inside Stitch's Great Escape... that's a few minutes of our lives we'll never get back.