What features are considered best for dive fins?

One feature is the foot pocket, which would be full foot or open heel.  With the full foot, your entire foot would slide inside much like a shoe whereas with the open heel, the heel part of your foot would be exposed but held firmly in place with a strap.  Most dive instructors recommend the full foot simply because it fits better and makes it easier to glide through the water.  The strap is also important when buying scuba fins.  The goal is for the strap to keep the fin in place, which is why the adjustable heel straps with side buckles are considered the best.  Then, fins are designed as a split or non-split with the split design being the newest.  In this case, the fin is split lengthwise in two parts for better propulsion.