Jared Spool writes an excellent article describing how to give a good design critique.
A well-done critique is a way to step away from the specifics of the design process and better understand how to create great designs. We do this by starting with the current design and asking "What is it we're really trying to do here?" and "How close are we to doing it?"
The design crit is one of the most valuable parts of the design process to me. Interaction designers and information architects without design or art backgrounds often find themselves new to the critique, but over time learn that the practice is a necessary aspect of evaluation and assessment of the work. The dialog, reframing, and perspective offered in the critique really helps the designer working in isolation. Spool offers some good advice for how to frame comments positively and respectfully, how to develop a healthy environment for critiquing, and how to use the crit to keep designers thinking about possibilities and alternative solutions.