Giving design feedback is hard, but with a few quick tips you do it like a pro.

The Design Feedback Sandwich.

The best way to keep designers happy is to use what I call the Design Feedback Sandwich.

1. Start by telling your designer a few things you like about the design. 
2. Next, tell the designer which pieces are problematic.
3. Finish by telling them something positive thing about their work.

Validating the positive bits keeps up morale. Remember - people are more creative when they're in a good mood.