Since you want people to sign up, try to re-arrange things so that the sign-up section is above the fold (i.e. front and center, so the user doesn't have to scroll to see it). Condense or eliminate the orange header bar, header and "about" section to make room.
Having all of those subreddits in the listbox doesn't work, so that UI needs to be re-thought from scratch. You'll probably want something like a two-panel multi-select widget. Probably show only the most popular subreddits to start, with the ability to search for others. Hope this helps.