I initially thought up this thread after posting something about using saves as defense as a 3.5 house rule. It was pointed out to me that this idea already existed in Unearthed Arcana. When I went looking through UA, I also found an option for no skill points, which reminded me of the direction 4.0 had gone.
So I got to thinking, what exactly was invented for 4.0?
Some stuff is of course original by the most narrow definition, but don't really count toward the question. For instance, I immediately discounted Dragonborn because that's just a set of modifiers, abilities and fluff applied to "generic pc capable race". That's not the sort of thing I was looking for.
Without access to my books, I could only think of one thing; minions. Veekie rightly points out that other games also have had this concept for some time. I played in one steampunk type game that had a minion-type system where the minions acted as a sort of ablative armor for bosses... used all d6s... can't remember the name of the game though.
Perhaps I should have worded it more carefully. I'm looking for mechanics that were newly introduced to D&D during the 4.0 era.
I would count minions as something newly introduced to D&D during the 4.0 era. [edit: Forgot about oslecamo mentioning MM5]
Dice Recharge mechanic for monster powers is found in the Dragon entry for 3.0, so that's a no-go.
I guess I simply don't remember enough about Milestones to judge them.
Magic Item's recharging based on encounters that day is something I also forgot about.
Now, don't get me wrong here. I'd rather see something listed with justification of how it's new than have a poster omit something for fear of it being labeled "too similar to mechanic X from 3ed". Sure, I'll classify certain mechanics that way, but that's just one goblinoid's opinion.
For instance, I'd say the skill check system nearly makes the cut, but not quite. It's just the UA version slightly modified. Having said that, I recognize the argument that it's not identical so some people will feel it is a new mechanic. I'm not going to attempt to "correct" that line of thinking. Plus I'd say the UA version is actually a better mechanic.
This is the same thing which disqualifies 'per encounter'. While technically not 100% identical, the ToB recharge mechanic is not exactly the same as 4.0 (IIRC) it's close enough that it doesn't count as introduced new to 4.0. It nearly makes the cut for a new to 4.0 mechanic but not quite.
Mostly it's just an interesting diversion of a conversation.