What will you be cooking? If you'll be cooking large pieces of meat such as whole roasts or chickens, make sure you get an oval slow cooker that can accommodate them. Round slow cookers, on the other hand, are just fine for soup, stew, chili, pasta and the like.
What capacity do you need? If your household is smaller -- up to a few people -- you can get away with a smaller slow cooker that's less than 4 quarts. A family of four is going to want at least a 4- to 5-quart slow cooker; larger groups will probably want at least a 6-quart model. Remember to consider how much space you have, too. Models with larger capacities have a larger footprint, so you'll need enough counter space (or storage space) to accommodate them. Whatever size you choose, note that adding too much or too little food can lead to less-than-ideal results.
Will you be taking the slow cooker on the go? If you're a regular at neighborhood potlucks or game-day parties, you may want to bring your meal contribution in the slow cooker so you can keep it warm during the event. Be sure to look for a model with a locking lid that seals completely. A retractable cord and easy-to-grip handles are also handy features on a portable unit.
If your insert or lid cracks, can you replace it? A quick scan of owner reviews shows that the inserts and glass lids are the most vulnerable parts of a slow cooker. See whether you can buy replacements without ponying up for an entirely new slow cooker.
Will you be around to monitor your slow cooker? Many busy families like slow cookers because they don't have to babysit them, but you don't want to run afoul of food safety guidelines by letting food sit too long at low temperatures. According to the Food and Drug Administration, bacteria can be a risk once temperatures drop below 140 degrees. If you're working long hours, opt for a programmable model with a longer timer that will switch your food to "warm" once it's done -- it will keep your food at a safe temperature but reduce the risk of overcooking.