Expect heavy crowds in Epcot's World Showcase, especially in the evenings and on weekends. The Festival Marketplace booths are laid out in between all the pavilions there. Expect to drop a fair amount of cash even with the use of your snack credits, especially if you enjoy adult beverages. Food and wine items are sample portions that generally cost between $2-5 each, though some mixed drinks and sparkling wines can cost as much as $10.
I think the must-try items include the Lettuce Wrap from South Korea, the Spicy Hand Roll from Japan, the Tuna Poke from Hawaii, the Lobster and Seafood Fisherman's Pie from Ireland, both tacos from Mexico, and the Sparkling Pomegranate Kir and Eiffel Sour Cosmo Slush from France. I posted photos of some if these items on this thread:
http://forums.wdwmagic.com/threads/...-festival-marketplace-kiosk-offerings.870121/
As far as items to stay away from, that's even more subjective than what's good. I advise exercising extreme caution at the Singapore booth, though. I thought the mahi mahi was totally bland, and the Singapore Sling was so strong it tasted like paint thinner.