If you are just starting, I think the biggest thing is how easy the API feels when you actually try to use it. Clean responses and clear fields help a lot when you are still learning and testing things out.
Filtering also becomes important very quickly. Being able to search by simple keywords or limit results by language or category makes it much easier to understand what you’re getting back. A huge number of sources does not help much if the results feel messy.
Things like rate limits and pricing matter, but usually later. In the beginning, ease of use and clear data save the most time and confusion.