From my travels I have learnt-

Hard case rather than soft
Better quality rather than cheap (used one suitcase once- the trolley handle was broken and unrepairable- good bye suitcase)
Four wheels rather than 2 (makes it easier when you are standing in a line, just push along, no need to tip).
Don't go too big (you will never be able to pick it up to carry upstairs)
Zip with rubber bumper rather than clip (clips fail more easily and you can get zips reaplaced. However it must have a rubber bumper to help protect the zip)