A couple of things to look for are:
The pull handle - make sure it can lock in place (airport luggage handlers sometimes throw the bags by these handles and if they do not lock in place they can slide out and get damaged very easily) Also make sure that it is long enough for your height. If it is too short, you will feel it pull at your back when the back is full. Also make sure the handle slide is either inside the suitcase or if outside it has very strong protection. I prefer mine inside (less likely to get bent) and I pack socks and smaller items around them.
The wheels - make sure they spin freely (ones like they use on skateboards or similar with ball bearing are very good) ALso make sure they are protected by the bag itself or have some sort of plastic by them. The bag will be sturdier the farther apart the wheels are but you DO NOT want them to stick outside the case dimensions. They have more chance of getting knocked off if they stick out. You want the wheels flush to the case or maybe 1-2 inches from each side.
Zippers - make sure they slide easy around corners. Rounded corners are better than squared off corners.
Outer materials depend on your budget. It has been a few years since I worked in a retail luggage store, but if I remember, Swiss Army/Victorinox has nice mid-grade luggage. It is not the cheapest and not considered luxury, but it was well made and designed. And it what I have used for the last four years of traveling with the airlines. I think a 24" case should be in the $250-$300 range. Anything larger than that and you run the risk of going over the 50lb airline limit on luggage and no one needs extra fees these days.
Hope this helps. You can find nice luggage at discounted rates at places like TJ Maxx/Marshals, too. Just look at the handles, wheels and zippers as that is the most important. Any material can get slashed easily if someone want in your bag.
Cheers!
Lisa
