Some suggestions for ensuring your feedback is constructive:

• Be gentle. As long as you get the message across, there is no need to be harsh. Receiving even nicely-worded negative feedback can be difficult.
• Be specific. Provide as many details as you can (without deanonymizing yourself, should you wish to remain anonymous). Bad: “Your meetings are wasting people’s time.” Good: “When you arrange meetings, sometimes they are ineffective because there isn't a clear purpose and you don’t stop people from going off-topic.”
• Give positive feedback sometimes. Just as the recipient may need your feedback to change behavior that bothers you, they may not be aware of the good aspects of their behavior, and even if they are not blind to them you can encourage them further by giving explicit praise.
• Pick actionable things. Try to give feedback the recipient can act on.
• Don't abuse. It's easy to be mean when anonymity protects you. Remember that the goal is helping the recipient, not making them feel bad.