In Discussing Design: Improving Communication and Collaboration through Critique, Adam Connor and Aaron Irizarry explain what separates critique from other forms of feedback:

Good critique is comprised of three key elements:

1. It identifies a specific aspect of the idea or a decision in the design being analyzed.
2. It relates that aspect or decision to an objective or best practice.
3. It describes how and why the aspect or decision work to support or not support the objective or best practice.