Beyond the design you should include your names, the date and the city and state of your wedding, you don't need the actual location. And of course you want your guests to know they will receive an actual invitation in the future so include a line like "invitation to follow." If you've set a website put on there too!
Sending Save the Dates at the right time is also important, you can send them up to 6 months in advance and earlier but anything after that will come so close to your wedding invitation that it may be an unnecessary expense.
So if you need it, send it and make sure you have a little fun here, give people an idea of what to expect. Is your wedding going to be the party of the year? Then make it fun. Are you planning a romantic and elegant wedding? Then you should nod to that feeling with your Save the Date too. Consider something different than a card in an envelope...postcards can be cool and cost effective, a letter is also a beautiful way to fill everyone in, magnets are always popular choices too.