I had an interesting discussion about this, it's a great question and really got me thinking about what I would have found useful in that position. With the obvious suggestions like a good grasp of the animation principles and a growth mindset (always learning, improving and pushing for feedback) there's one that popped into my mind:
More specifically, the ability to choose good reference and deconstruct it.
First of all for those who are new to this, I want to clear the air about the use of reference: Using reference isn't cheating. It just isn't. Most artists use reference to some degree, even the greats. Personally my work is always better when I have at least something to look at and refer to. There are some gifted people out there who have the ability to animate anything straight from their imagination (maybe you're one of them) but the rest of us should make reference an integral part of our work-flow.
Also note: there is a difference between using something as reference and plagiarism. Don't be that guy.
Bad reference can set you off on the wrong foot, so how do you choose good reference? First of all, it depends on what you're looking for. How do you even decide what you're looking for? Well, maybe you just want a rough idea of timing, a few poses to figure out foot placement, camera framing or how an arm settles to a stop after an action. Your reference doesn't have to be totally perfect and you can use just small parts of it. I like to gather multiple reference videos and Frankenstein them together in my favourite video editing program to get exactly what I need.
The great thing about reference is that you can use it to try out lots of ideas quickly and even get them approved before you start animating. This saves loads of time by minimizing the chance of redoing animation because the idea isn't right for the project and ensure that you are always exploring your options and making the most interesting choices for your animation.
Here are a list of sources to start you off on gathering reference:
Creating thumbnail sketches is basically a source of reference. They help serve as a guide when you start animating and they are a source of solutions for any animation challenges you may encounter. The great thing about them is that you don't even have to be good at drawing, just a simple stick figure will suffice.
Film it yourself
If it's something fairly simple that can be done with some degree of competency then grabbing a camera and shooting your own reference is a good option. If you're not comfortable in front of the camera then grab a willing friend.
If you can't climb a mountain or do a tornado kick then it's better to leave it to the professionals and search YouTube/Vimeo for reference. There's a lot of content out there and it's quite easy to find an example of what you need. Once you've found what you need you can use something like keepvid.com to download the video and scrub through it with more animator friendly software.
Having a bizarrely encyclopaedic knowledge of movie scenes and Game Of Thrones tends to pay off in the hunt for reference (now I can justify binge-watching it when the latest season is released). It's usually the first thing that pops into my mind when starting something new and is a good launch pad to kick things off.
If you have any questions, comments or feedback then I'd really like to see it. Post it in the section below and I'd be happy to help.