Personally, I find I need wheels on my luggage - modern airports are huge - and I like to travel light, with the largest bag that will fit in the overhead compartment plus a normal carry-on backpack that can fit over the handle. (lots of airlines are charging for checked bags) I can last comfortably three weeks with this setup.
I like the durable soft sided bags that can expand with a second side zipper for situations where I need a little extra room for gifts. I always had good luck with Swissgear, but weirdly my most resilient luggage has been an off-branded Nautica bag I got at Marshall's for $50. It's still going strong after dozens of trips.
I don't like hard sided carry-on bags as much, because they get dented up and aren't as flexible for fitting into small spaces. They are great tho for larger pieces that will only be checked. I have one for longer trips/cruises. They are much safer than fabric checked bags since they can't be ripped or slashed by a razor in baggage handling - something that's happened a couple of times to me with bigger bags on international trips.
I usually look for a few key features. I like the wheels to be exposed as little as possible and of very good quality since they get the most abuse. I also like the entire extending handle mechanism to be internal and protected from impact. Things that stick out tend to break off. That's why I prefer the flat fabric side and top lifting handles that are flexible and don't stick out. I also like the luggage to have a reinforced panel on the back - like a skid pad - to protect against abrasion when the bag goes up a curb. And the zippers should be lock-able, high-quality, heavy-duty zippers with durable crush-proof pull-tabs that can easily be replaced with something else if they break (they will). Any extraneous decorations will get destroyed, and esthetic nuances that look good in-store will be quickly overlooked.
I figure the average bag lasted me about 2-3 years, some much shorter and some much longer. Certain destinations are much harder on luggage than others.