For every startup to succeed Product Market Fit is an essential part of strategy. Though there are lot of theories and assumptions on finding your PMF but in my view your product's transition from "Nice to Have" to "Must to Have" is only thing that clearly defines that your product has found great fit for its customers. Though Functional need of customers is at the core of any product but only emotional need of customers is the thing that makes a product truly successful. This is when they become your voice and this is when "Getting Viral" becomes a notion of your success. In my view bringing out a product with features that could become Must to Have for customers is best ever product strategy. When you are done with your MVP I will suggest you should must ask one question from your users "Which features of our product you will like to retain". It will provide you enough information for iteration of product features and finally arriving at product that customers will defintely love.