They Don't Require Special Detergent
High efficiency detergent is a special type of non-sudsing detergent. Front-loading washers work in a way that doesn't require a lot of suds. In fact, too many suds in a front-loader could actually damage the machine.
High efficiency detergent can be more expensive than regular old laundry detergent. There's also less variety than you're probably used to.