One of my favorite mechanics from 5e D&D is the Advantage/Disadvantage system. It is a simple tool that works well to automatically make things harder or easier for the player. It replaces the need to figure out pesky things like how much penalty does this level of darkness have, how much benefit can my friend add to this check, what happens if I'm covered in oil and try to handle a smooth glass cylinder?
Liberal usage of Advantage/Disadvantage can speed up the game. And it is an easy system to use, or tweek, to fit your own needs.
How It Works
The core to Advantage/Disadvantage is figuring out if a character is in an Advantageous position to do the check, a Disadvantageous position, or a neutral position.
If things are looking up for the character, they roll 2 D20s instead of 1, and they take the die that has the better result on it. If things are looking down, they roll 2 D20s instead of 1 and take the die that has the worse result on it. If things are neutral you just roll one die as normal.
And that's it.
Cancel Out by Choice
By default any advantage cancels out any disadvantage and vice versa. So if you have three things that should give advantage, and one thing that gives disadvantage, you have a neutral roll. This is easy enough to tweak or change to fit your needs though. Make them cancel out at 1:1 if you want, and then go wild with the different status ailments and benefits.
How To Steal
The hard part about stealing Advantage/Disadvantage is that it works best in a system where you roll one die. If you roll a dice pool it can seriously add a lot of time to making a check. Though most systems, dice pool or not, have a Lucky mechanic that does that anyhow.
How To Use
Advantage/Disadvantage is a great way to just short hand bonuses and penalties to a roll. You don't worry about finagling things. Flanking an opponent doesn't reduce their AC, or give them a flat footed penalty. It just gives advantage. At the same time, you don't have to worry about how much penalty you have when tied up in spider silk while trying to climb a wall. You just roll with disadvantage.
That short, easy solution to handling the more intricate nuances is where it shines. It doesn't remove all chance, it just makes it harder. About 25% harder if you just convert the penalty to passive scores it gives in 5e (+ or - 5 out of potential 20 results on the D20.)
Simple. Elegant. Versatile. That's a mechanic I can really get behind.