Hi Laura
Service is part of it, but what service aspects are unique unless something goes wrong on the trip? Price is important to me, as is the addition of bonuses (in order of preference, shipboard credit, onboard vouchers, coupon book) that tell me my business is appreciated.
I expect that my TA gets back to me promptly and go the extra mile to find me the cabin I want and is understanding when I want to research multiple options ...