I think keeping some dependence on quantity is desirable, but that scaling linearly with number of posts weights it too heavily compared to variation in number of upvotes (I proposed scaling with roughly the cube root of number of posts in my explicit formula suggestion elsewhere in the comment thread).
A good system for finding old content. Often old posts or comment threads represent the best content on a given topic, but if there isn't a way to browse by topic they're basically lost. This makes me less excited about engaging in the first place -- vice versa a sense that something will persist is a draw in a venue for circulating ideas.
This is one thing where I think the forums/subforums structure of old UBB boards is actually superior to most more modern replacements (although such boards have other significant problems, most notably lack of threading).
Here's an example functional form which is the best guess from the top of my head at creating this effect (but I'm giving as an illustration of what to pay attention to rather than a claim that this precisely should be used):
K = (U - 3D) * P^0.3 / R
Where K = Karma U = total (weighted) upvotes D = total (weighted) downvotes P = total number of posts+comments R = total number of reads of your posts+comments
One variation of karma system I'd like is the ability to rate posts as being exceptionally good (probably taking more than one click, to introduce a trivial inconvenience so that it isn't used all the time like five star ratings are). This would give more ability to pick out very useful contributors from small numbers of posts.
You talk about using karma thresholds for various things. But traditional lesswrong style karma screens more for quantity than quality of posts, and this would remain true of a version where you weight people's upvotes and downvotes. I suggest looking for versions which filter more for quality (while not creating too much disincentive to make additional posts/comments).
I'm in favour of this, but I think it would be even better to give high weights by fiat to some trusted users, in a way that grounds the whole Karma system in something, and makes it harder for low-quality bubbles to self-perpetuate.