Apparently the most requested feature is to lower the minimum score threshold. This restriction made more sense when the keywords filter was not available: A subscription with a score threshold of less than, say, 200 points (this number was fairly arbitrary), ended up in the user being literally spammed by the service -- While testing the service, I roughly estimated that a score of 100 points is equivalent to ~1.5 emails every 15 minutes. The usability, cost, and scalability of this approach is probably not ideal.
However, now that custom keywords are involved in the criteria, I do feel like the minimum score can be lowered. Perhaps even removed, if the user has selected 1+ keywords (they are optional). I'll follow up on this feature in the GitHub issue, https://github.com/ichinaski/hnnotifications/issues/2