There's a checklist in my head, but honestly, it's different for every app. On one hand, I need the basic stuff to work perfectly – no crashes, a clean library view that doesn't lag when I add hundreds of titles, and a night mode that's actually dark amber, not eye-searing blue. The font and spacing options are non-negotiable; if I can't tweak it until the page looks exactly right, I'm dropping a star immediately.
But the real separation between a four and a five-star review for me is the reading ritual. Does the app remember my place across devices without a hiccup? Can I highlight a passage and export my notes easily? I once gave a full five stars to an app simply because its page-turning animation was a subtle, satisfying curl instead of a jarning slide. Those tiny, almost imperceptible details are what make me feel like the developers actually read books themselves. I dock points aggressively for intrusive ads or paywalls that pop up mid-chapter.
My last star is reserved for community features. A comment section per chapter that isn't a toxic wasteland, or a way to see what highlights other readers found poignant. It turns a solitary act into a shared one, and that's magic.