How tall is your character? How much does he or she weigh?
As we discovered in the most recent Glantri session, where the party thief (a ten-year-old urchin) proved to be too short to keep her head above water in the city sewers, such measurements can have a real impact on play, to the point where most editions of D&D provide tables for randomly generating such vital statistics. This plays off the old-school approach of trying to resolve one’s interaction with the game world through description and logic rather than through the intermediary of a skill system.
Off the top of my head, benefits for various physical extremes include the following:
Tall characters have an easier time climbing, in that you may be able to pull yourself up to a ledge (possibly with a boost from another PC) rather than needing to find handholds. And, as noted above, they find it easier to keep their heads above water!
Short characters function better in areas with low ceilings, like your typical dwarven or goblin mine. They can march around perfectly well while the big’uns are suffering penalties to their actions from being constantly hunched over.
Light characters are easier for the party to lug around if need be. Whether you’ve been knocked unconscious or simply need to be hauled up a shaft on a rope, it’s easier to get you where you need to go if you only weigh 80 lbs soaking wet. Such characters may also be less likely to trigger pit traps, break fragile rope bridges, and the like.
Heavy characters may be harder for monsters and NPCs to pick up or push around, such as when you’re fighting on the edge of a cliff and someone wants to shove someone else over the edge.
Naturally, all of these things require adjudication by the DM. But since they don’t involve abstruse game mechanics, it’s easier for players—even casual players—to argue their point of view on an even footing, and as long as there isn’t an adversarial dynamic at the table, the players and the DM should be able to agree on these sorts of things.*
* When in doubt, I believe the DM should concede points about the game environment and state of play to the players. This is probably fodder for a whole blog post in and of itself.