Our campaign has over the past decade wound up with our own particular defining characteristic for halflings, based on the original 1e racial traits, and supported nicely by the 4e halfling stats:
Halflings have a ridiculous capacity to survive the most deadly circumstances - circumstances which they usually bring upon themselves through their utter lack of self-preservation instincts, which in turn is due to their capacity to narrowly escape danger... and so on, in a vicious circle back to the dawn of the halfling race.
Or in the words of the progenitor of this archetype, who played Boen, the halfling in our 1st ed Temple of Elemental Evil game, and the Bugs Bunny to the party ranger's eventual Elmer Fudd:
This is the direct result of generations of halflings jumping into proverbial meat grinders. The survivors breed, rabbit-like, producing countless children, many of which are hale enough to survive their own stupidity, misfortune, and accrued enmity. The cycle repeats itself, like a beautiful cosmic clown car perpetually unloading its passengers into the perplexed mouths of no-longer-hungry lions.