I try to get by with hand luggage all the time so go for 2 criteria – lightweight – the lighter the case the more stuff you can take – some of these hard case/heavy duty cases are ridiculously heavy and you simply don’t need the robustness for hand luggage. Also they have crap internal volume due to the more heavy duty nature of the exterior shell.
I like external pockets especially for laptops as you’re always wanting to access stuff and I don’t want to be always opening the main part of the case to get stuff in and out especially when you’re in a queue at security and having to fish your laptop out of your main case.
Currently I’m sporting an IT case that claims to be the worlds lightest case in the world. It is certainly lightweight and plenty spacious with plenty of external pockets and an external laptop pouch so really convenient for security. Can easily do a week long work trip with it without having to employ the British army method of getting 4 days out of a pair of undies.