I've used lingering injuries in play, and provides me the opportunity to drop cybernetic magic items. Cue the Hand of Vecna...
One of my favorite rules is Shields Shall be Splintered. You can sacrifice your shield or weapon to negate a single attack.
I'm working on giving the shield a bit more love in my Moldvay/Cook/ LL hack. The shield really doesn't get the prestige it deserves. The...
trollsmyth.blogspot.com
In a recent game, the party was getting beat up by a white dragon. Two characters were down, and the paladin was protecting his downed companions, but was as single digit HP, and the white dragon rolled a crit on it's bite attack...
The paladin's shield was a key point in his backstory, as it had the names of all his dead family members engraved upon the back of it.
The players decision to invoke the rule was the a key part of the party avoiding a TPK, and was a huge story moment as well.
I've used 13th Age's Escalation Die as well. It can really speed up big, sloggy fights, and you can use nat 20's and nat 1's to move the die as well.