Shortly, professionalism, because it includes everything a Dallas roofing contractor must be: reliable, registered, insured, experienced and focused on quality services and on its customers.
When you are looking for a roofer, the most important thing is to make sure you are dealing with a licensed company, registered in your state and fully insured. If these basic requirements are not met, there is no point in going on with evaluating that company. Just remove it from your list and go on. The roof is far too important to risk leaving it in the hands of a so-called roofer that lacks proper credentials.
Also, allowing a roofer that is not insured to work on your property without can put you in situations that you definitely want to avoid, such as making you liable for the injury of a worker or for property damage.
A good roofer also have a good reputation that you can check by contacting previous customers and asking them about their collaboration with the company you plan to hire, or by checking reviews and testimonials on social media, Better Business Bureau etc.
Hiring unethical or inexperienced roofers can incur high costs, so make sure you find a real professional to avoid burning up a hole in your pocket.