… and who you say it to.
You’re in the office every day. You develop opinions about your coworkers. Cool. That’s human. “John is a smart guy but he’s not the most motivated.” “Linda is articulate but she likes the sound of her own voice.” Those observations are fine to make but try your best to keep them in your head while at the office. Even if the person you tell your opinion to doesn’t tell anyone else in the office, you are impacting their ability to develop their own opinions.
The exception to this rule is if you are discussing someone’s flaws with someone else who can help them improve upon those flaws. At that point it’s alright to say to a manager “Hey Nick, John has a lot of great ideas but he’s having trouble implementing them. Maybe you could have a talk with him about it so that we can be a more impressive work unit.”
Make professional development a priority.