Go With The Flow
Airflow through a case is important for keeping it cool, because air that stays in one place is prone to heat up rapidly. That said, before looking at the cooling abilities of cases you are considering for purchase, you should also consider if the airflow through the case is likely to make a difference. If you're building an HTPC with a low-energy processor, a small motherboard, and a low-end video card, heat isn't much of a worry. As long as you have an exhaust fan you're probably good to go.
But if you are building a more powerful PC, and airflow does matter, then consider where the fans on the case you're considering are located. Preferably, you'll want to have at least one intake and one exhaust. If you're going for a gaming machine or a workstation, then you'll want to look for two intakes and one larger (120mm) exhaust. And if you really want good airflow and cooling, look for a case that keeps the PSU at the bottom and provides and exhaust fan at the top of the case.
Cooling is one aspect of a case that can be measured, so read up on any reviews you can find and take their findings into consideration. Remember that cases with poorer cooling can be made more acceptable with the addition of more fans, but that will make those cases louder. On the other hand, modern PCs, even high-end ones, run relatively cool, so it may be worth buying a slightly warmer case if it does a better job of fitting all of your other requirements, particularly if one of those requierments is a not being too loud.