I disagree with this part -- I think people should get credit for time spent lurking.

I would agree in general, except remember we're not giving prizes for not being a new user. Someone who lurked for a year and then made a comment is exactly the sort of person we don't want to scare off by being too harsh the first time they comment.

Back in the OB days, I lurked for several months, reading through the archives, and my first comment was quite brutally shot down in one dismissive sentence by Eliezer. I didn't comment again for over a year.

Number of posts is, in my view, a far more reliable way to quantify a person's dedication to the community than time-spent-reading. It's far more likely for a long-time-reader commenting for their first time to be alienated or intimidated by downvotes and criticism than a short-time-reader who has been commenting prolifically.

How do we know they're lurking and not just dropping by every few months?