Posts are linkable, comments to posts are not. (ERRATUM: they are also linkable)

Compounded with the fact that comment threading is limited to two levels and comments are loaded in small batches, making even the browser in-page search function useless, it makes following discussion with hundreds of comments impossible.

However, one of the things that Facebook does right and LW/Reddit does not, IMHO, is that votes on Facebook (well, there are only likes, actually) are public. Even if there were downvotes, making them public would make it socially harder to pull the mass downvoting sprees and other manipulations that plague LW and Reddit.