My suggestion is that top-level posts should be made when there is at least a paragraph of commentary on the link - if the poster does not have that much to say about it, link it in the open thread.

Self censorship is high already. Top level link post even without a description is okay because a comment with a description will invariably be written and voted up to top.

In the end, let the voting system do its job. Every time you post, you risk your karma. Top level post is basically raising the stakes.

How about a top-level "put cool stuff in replies" post every so often?

I'm imagining it as a home for anything good (links, books, video....) which isn't necessarily going to lead to significant discussion. As with open threads, if discussion happens anyway, it can be a subject for a top-level post.

I suggest a new subreddit with a ranked list of links and self-posts. A consequence of the current convention is that links are fine when they are above a certain level of goodness and that barrier is extremely high. If anyone wants to experiment, you could try other Ted talks, though it will be hard to control for the amount of text you write to go with your post.