Is there a certain characteristic considered most important when choosing a snorkel?

The answer is yes, the barrel, which should be long enough for easy breathing but not too long in that as the barrel fills with carbon dioxide that is inhaled, making breathing difficult, which can actually cause death.  A good rule of thumb is that the inside diameter of the snorkel should be about .75 of an inch.  Anything thinner would make it too hard to breathe, leading to hyperventilation.  Anything thicker would mean the barrel is too large making it hard to attach to the mask.