It's important to remember that the heal-to-50% proposal is a starting point for discussion, not the game's final destination. I like the intent of the proposal, and I hope we see something like it in the published game. However, I have a counter-proposal which I believe retains the spirit of Mike's suggestion, but makes more in-game sense.
Characters can be injured, a status effect that kicks in when a PC is reduced to or below a given hit-point threshold. By default, the threshold is 0, though this can be dialed up to 25% or 50% of max hitpoints for a more demanding game, or left out for an easier one. An injured PC has their max hp reduced to half its original value, and can only overcome this status by restful sleep typically found in civilized areas. Sleeping rough while injured cannot restore a PC above half hitpoints.
In the basic or standard game, being injured grants no other penalties, though the players may choose to roleplay how their character is hurt, e.g. by describing how they limp along and grimace with pain if they have to hustle. A lingering wounds module could enhance the injured status, adding rider effects or mentioning different ways to get injured (e.g. a critical hit from a monster 1 or more sizes larger).