One of the most important things about writing is voice. I would argue that many people will forgive all sorts of flaws in a story if the voice is just right.
Think about the stories that you love - why? What is it that makes you love them? I would argue that even though some may say the characters or the plot, it really is the voice - what the author gives the character to say in just the right way.
Capturing the voice in writing can be tricky, because it can often sound like the person talking. We have to take the time to really think about our characters - how old are they? Where do they live? What has life been like? What do they do and what do they want and how is this communicated?
One of the best ways to get a feel for voice is to eavesdrop. Yup, on as many people as possible. Listen to teens (most don't talk in text), adults, people from different countries with accents, married couples, newly in love couples (nope, they aren't always the same unfortunately), sophisticated - EVERYTHING - because that is how real people talk. And the most essential part of creating a character is making them real.
How have you found helps you identify voice? What do you do to make sure characters don't fall into the trap of all sounding the same?