At Origins, I played in a couple of D&D 5th Edition playtest sessions, for a module and for the online tools. Two mechanics stuck out for me: the new system for preparing spells and advantage/disadvantage.
Advantage and disadvantage are simple to describe and powerful in their implications. If you have advantage, make the roll twice and take the higher number; if you have disadvantage, make the roll twice and take the lower number. Done.
3rd Edition had a similar intent with its “+/-2” default rule. If the DM was not sure what sort of bonus or penalty something imposed, just go with “2.” That is a 10% difference on a 20-sided die. How does “advantage” differ?
Quite a bit. Several people have run the numbers (I think “enumerate the 400 possibilities” is a better method than running a simulation). As noted, the effect of advantage is small at the extremes and huge in the middle. If you are nearly certain to succeed or fail, advantage is +1 or +2. If you have a 50/50 chance, it is +5. Out of 20, that is really, really big.
Players will also feel advantage and disadvantage very strongly because of the perceived gain/loss of the second die roll. If you roll two 18s, “eh,” you say, “advantage didn’t matter.” If you roll an 18 and a 2, that’s a success with advantage and a failure wit disadvantage, and you can see fate hanging in the balance of that mechanic. It’s a psychologically powerful factor.