Licensing of software. The computer programs that make these features possible might belong to the developer or, more likely, to a software company. Have the designer help you obtain licenses to use any necessary software so you can move your site to another host if you choose. Make sure you get copies of the software along with your Web site files. Have the developer agree in the contract to obtain permission to use any copyrighted material, and warrant that all other work is original and doesn't infringe on the copyrights of others.
Some developers offer low initial fees but raise their prices for updates.
Or you could be at the developer's mercy. If the developer hosts the site, the monthly fee agreed to in the contract might include a certain number of updates or changes. Make sure the server hosting the site has enough bandwidth to carry expected traffic. While it's common for designers to host the sites they develop, you may not want to keep your site on their server forever. It's important to negotiate separate contracts, one for developing and one for hosting.