Hey Dan, welcome to Animatedbuzz! Hopefully this question accumulates many different answers from a range of experiences and perspectives.
So I think this depends on what specifically you want to do for a career. I can speak for CG/2d character animation since that is what I have experience with.
For a character animation portfolio, the most important thing is always to make a great impression. Often times that means putting your best work in, and your mediocre work out. Sometimes we get the impression that putting absolutely everything we've ever animated into a portfolio communicates something good, that perhaps we are very productive. but I think most studios will appreciate a shorter, high quality reel that gets to the point. We can tell right away if an animator has a skill set that will yield high quality productivity in a studio. So this means that all you need is probably a minute of character animation in your reel that demonstrates your ability to create believable, sincere, entertaining performances.
So to sum it up, they will look at acting, ideas, and your understanding of mechanics, which means the quality of your character's movement in relation to weight, balance, pose, etc. Also, they will look at how well you can design poses. Pose appeal is especially critical at Disney. So a drawing background or just a general design sense can come in handy for crafting appealing poses. Idea generation is also critical. This means that you need to showcase your storytelling skills. Setting up interesting and entertaining gags/jokes will be a great asset for any studio. But you need to demonstrate that you can do this clearly and in an interesting way.
On Animatedbuzz, you will have access to Coaches who work in the animation industry. You can consult them on works in progress, or any demo reel you might want to submit to a studio. So you will be able to get great advice from working professionals for a fee which they set. Coming very soon!
Hope this helps, good luck!