I'd say 'prioritization', especially in the beginning when users do not know exactly what they need/want.

In the mobile app market, a financially-viable or organic (non-paid) long-term marketing plan is the hardest. First testers can be hired. First users can be paid for with Google Ads/Apple Search Ads, etc. Depending on the competition, sustained user-base growth can become a next-to-impossible problem to solve.