On average, a user spends less than 30 seconds deciding whether to keep or delete an app they've just downloaded. Thirty seconds to justify taking up space on their phone, learning a new interface, and trusting you with their data. It isn't much, and it's all the time you get.
Asking for permissions the moment it opens
Imagine walking into a shop and being asked for your phone number, your address and access to your calendar before you're even allowed to look at the products. That's exactly how it feels when an app asks for permissions before the user has understood what it's even for.
The rule is simple: only request a permission in the context where it's useful, and explain why you need it. Never before.
Loading that takes too long
If your splash screen stays on screen for more than a second and a half, a significant share of your new users have already lost patience. On mobile, speed isn't a feature, it's a right. A slow start makes a product feel sloppy, even if everything else is excellent.
An interface no one can make sense of
You know your app inside out. Your users are seeing it for the very first time. If the core feature isn't obvious within a few seconds, no tutorial, no explanation, you have a design problem, not a user problem. Test your app with someone who has never seen it. Whatever they don't grasp immediately is exactly what will drive your new users away.
Forcing sign-up too early
Asking a user to create an account before you've shown them why it's worth it is one of the surest ways to lose them on the spot. Let them explore, let them see what your app does well, then ask them to sign up once they've got a reason to want to.
Trust is earned, not demanded. Give your users a reason to stay before you ask them for anything.
The apps that keep their users aren't necessarily the ones packed with the most features, they're the ones that remove the most friction in those first few minutes. Every second shaved off loading, every deferred permission, every eliminated sign-up step improves your retention directly.