Carry-ons need to be lightweight. You will be toting it through the airport and hauling it up into an overhead bin. Some luggage-makers have even started producing carry-ons that are shorter and wider. The design change means you can still fit the same amount of items in the bag, but it will fit more easily in an overhead bin.
You can find lightweight hard-shell carry-ons, though many people prefer soft shell. Remember that a hard shell might be difficult to cram into the overhead compartment of the plane, whereas a soft shell gives some. The carry-on's components determine the weight, which means you want to find one with an aluminum, fiberglass or graphite frame. Nylon and polyester are popular lightweight fabrics because they can take a beating.
Features to consider
Whether you prefer a hard or soft shell, you might be tempted to purchase one that's labeled expandable. An expandable suitcase is nice because you can unzip a zipper to make it larger. However, if you expand and fill it, it may not meet the airline's size requirement to be considered a carry-on. That doesn't mean you shouldn't buy an expandable carry-on; it just means you need to be aware of and meet airline standards so you can carry it onto the plane. Because the wheels, zippers and retractable handle get a lot of use, make sure you check how those three items are constructed: the more durable they are, the better.
For smaller, laptop-sized bags, look for one that has lightly padded handles and an adjustable strap for those times when you need to throw it over your shoulder and race to your next gate. A no-slip pad on the shoulder strap will keep the carry-on from sliding down your arm. A strap and handles with soft padding makes it more comfortable to carry the luggage without being hard to grip.
For larger pieces, a sturdy retractable handle and wheels are key. They will allow you to roll the carry-on through the airport and stow the handle for storage. (You'll notice a lot of people use the extended handle as a convenient place to drape their coats so they don't have to wear them while waiting for their planes.) Side pockets are helpful because you have a place for your boarding pass and other items you need to grab quickly and easily.
Traveling with a carry-on is convenient. You'll save time by not having to check luggage or wait at the carousel after the flight. You can even save money by just using a carry-on. Let Sears help you purchase one that is lightweight and meets airline requirements.