Unit economics - what it'll take to make this work with healthy profit margins? This is crucial in case of online-offline startups, infra heavy products, or hardware products. May not be critical in consumer startups until they start monetizing.
Bottoms-up analysis of market - what will be your CAC (cost to acquire customers), ARPU/LTV (avg. rev per user/lifetime value) etc.
Both of these things are quantifiable and it'll answer an important Q - how much money will it take for the biz to reach whatever milestones that are decided.