Signature Skills is a term used by the D3 Team to refer to the most important, visual, significant skills possessed by each of the playable classes in Diablo III.
The team describes signature skills  as the skills that really define the characters, and it's through developing these skills that the design team comes to understand just how that character is to be played and what sort of space they will occupy in the game world.
For instance, the design goal of the Diablo III Barbarian is to make a huge, powerful, thundering brute of a melee fighter. So his signature skills are things that crush hordes of enemies in front of him, shake the entire screen, rip holes in the earth, etc. Wizards are deadly, reckless mages, so their signature skills are spells that wreak massive havoc upon monsters with fire, cold, and other elements.
Passive skills are never "signature skills." Even though passive skills can be essential and powerful, they're not visually impressive enough to be named a "signature skill."
Signature Skills Debut
The first reference to signature skills came at the WWI 2008, during the Denizens of Diablo Panel. In that discussion, Jay Wilson detailed the concept of signature skills and showed off a couple of examples.
- Jay Wilson: These are where we bring the design and art together. We focus on a small set of skills, and pick ones that are very cool to work on. By this we focus on defining a character's essential nature. When we start to work on a character, we can sit in a room and come up with 100 skills. Ideas for them isn't hard. We don't make them all though. We figure which are really cool and defining of that character, and those are the ones we pursue. We have a saying, "don't balance too soon". The goal here isn't to crate a skill that's fair and balanced gameplay. The point is to make a skill that people will think awesome. And that can be edited by worrying about balancing it too soon.
- I'm not trying to trivialize balance. It's important; but not too soon. Don't balance at the beginning. At the beginning you're trying to make the heroes as awesome as possible. One cool thing that happens is that we have an arms race between the characters as we design them. The first one is awesome, and then we make the next one a little bit more awesome. So we have to go back to the first one and up their awesomeness, and back and forth. It makes them better and better over time.
- Barbarian: Seismic Slam
- This is a skill where barb raises weapon over head. Slams it down. Anything in front of him gets rocked and destroyed. This skill was first skill we made for Barb, and one of the first we did in the game. Getting it right took longer than it did with most skills. We did many iterations. Animation especially. Had to make barb feel physically powerful. Traditionally we associate big bold movements with power. But if you want a game that feels fast and responsive, you want the movements to be really fast. Getting balance of fast movements and powerful feel. It's a big challenge in many ways to get that animation right.
- Once we got through all that, we didn't still didn't really like the skill. We got programming done and wasn't that great. Got effects done, and still not that great. Somehow didn't quite sing to us. Wasn't until we got sound that it was really awesome. Really fun. Key to that was faith to get through process. Once you do get through it, all the skills that come after that come much quicker. What's the Barbarian about? Look at Seismic Slam. With that he rocks the world with power. Next few skills we did almost required no communication between art and programming, since art knew what the barb was about. Knew what we were designing for, since we had a great example.
- Witch Doctor: Firebomb
- Interesting evolution of this skill. It was one of our first skills for him. Originally it was a sort of fireball spell; when cast it travelled in a direct line from the WD to the target and would hit anything in the path. It exploded nicely. Explosions are cool. Everyone likes them.
- Problem was, not really problem, but we've all seen that kind of Fireball spell before. The artists came back said we didn't imagine him that way. We saw Firebomb as more of a physical magic. Like concocting some alchemical recipe in a skull and hurling that, and it explodes.
- Once it was a physical projectile designers liked. It became a physical thing to throw, could go over walls. Become different tactics and use. Great example of how art and design can work together to iterate to make cool skill.
- Last thing artists wanted to do was put the image of the witch doctor into the fire effect. When fire goes off it's big form of his hands and face in the flame. We thought that idea was so cool that we put that into the graphics of his other skills. Horrify, Mass Confuse. Just shows how one skill can inform whole class.
- This is how we develop skills. Rinse and repeat this idea. First few are hard and time consuming, but the rest come easier and makes for a really cool character.
Sequence of Witch Doctor casting Skull of Flame, which was formerly called "Firebomb."
Signature Skills Listing
There's no full official listing of signature skills, since it's just a descriptive term the team uses to refer to some of the skills possessed by each character. Any impressive, active skill that seems uniquely tied to the character who possesses it is a good candidate for this expression.
The Exploding Palm skill was referenced as being one of the Monk's signature skills.
Mentioned as signature skills by Blizzard:
Skills that are generally regarded as unofficial signature skills: