I would like to humbly request that WordPress.com blogs have a feature that enables users to preview comments before posting them. As WordPress silently discards improper HTML tags, it is very easy to make mistakes and end up having a comment posted that is very different from what was intended.
There are already WordPress plugins to do this that would simply need to be enabled on WordPress.com, including Live Comment Preview and (better) Ajax Comment Preview.
I find there are several posts on this topic earlier, including this and this.