Reliability. Does the app reliably accomplish its purpose? App crashes are an egregious example, especially on diverse platforms like Android.
Another example: upload failures when trying to post photos or videos. Reliability issues are important because they cause a drag on retention as users churn off the app in frustration.
Craft. Do all the tiny details add up to create a genuinely polished experience? These bugs are rarely the most important one to fix or “launch blockers,” but they are essential to get right if you want the product as a whole to feel perfect.