A button to preview your post or comment reply before submitting would be cool. Markdown text formatting is confusing, terrible, bad, and doesn't always end up making my text how I want it. Being able to see how my post is going to look before I submit it is helpful. I'm a post editing fiend.
RES does this in a good way where it adds a display box under the text input box showing you in real-time how your post will appear.