I am not convinced by a low fee for commenting. The obvious spamming strategy is then to comment on notes by "high signal" users with a large number of followers, drowning out the real conversation. Your idea of user-specified threshold could get rid of the spam, but it also effectively prevents someone with no sats but great ideas to contribute to the conversation
Discussion
What do you think about using hashcash to prevent spam?
Fundamentally, spam is about leveraging very low marginal cost situations to multiply very low marginal benefits, so the necessary fee might be incredibly low.
On the user-specified threshold, it might slow down the emergence of someone with good ideas, but my guess is that most of us would have very low thresholds. On the other hand, I can imagine that there are some relatively big fish who might be more interested in using nostr if they can set a higher level and make engagement more remunerative.