What aspects of your product must be flawless before its release?

First of all, these are UX and UI.

Test usability with intended users to get input and make adjustments.

Make sure the product is compatible with a range of platforms and browsers.

Include mechanisms for users to report problems or offer feedback.

Content, design, and functionality.

Absolutely nothing :)

Basic functionalities and the UI

Stability, new products are usually more buggy, so I think it's very important to keep it stable so that you can get valid information from your seed users

The key feature of your product.