No matter how well designed your website may be, it won’t do any good unless people actually go to it. Most probably most of the traffic you attract will come from Google. A good SEO agency should be focused on people. Of course there a lot of technical stuff to consider, but in the end, people do business with you, not bots that’s why we give a big credit to customer experience.
Another important thing is the clarity of the communication. Google’s algorithm is complex. Thus SEO decisions often involve weighing pros and cons. This is where the value of the SEO partner matters, they should explain clearly and transparently their recommendations.