I would like to combine your two suggestions like so: Posts in discussion still earn 1 karma per vote. But as soon as a post gets at least five or so points it transfers to promoted. And then you get 10 karma per vote the post receives after getting promoted.
Posts in promoted are visible to people reading discussion, but readers can choose to see only promoted posts.
That way you have a smaller downside risk (if your post is received poorly you only lose one karma per downvote), but you can still get more karma if you write a substantial post that people like.
I really like the suggestion of making it so downvotes only cost 1 karma on toplevel posts. But it seems weird to have the marginal karma from an upvote suddenly switch from 1 to 10 as soon as you get at least 5 points.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.