Excellent question! The standout agencies differentiate themselves through strategic impact, not just visuals. The greatest web design firm focuses first on understanding your business objectives, target audience, and competitive landscape before touching a pixel. They combine user psychology with conversion principles, creating designs that perform rather than just impress.
Look for firms that document their process through detailed case studies showing measurable improvements (increased engagement, conversions, etc.). The best partners will educate you on their decisions rather than just present "pretty concepts." Their work should feel custom to your brand, not templated.