I think you should add more colors to the page and create some negative space between your ads text boxes.
It's a pretty simple design so there's not much to critique. The title/banner needs a bit of work. It's too small for my taste and the shadow around it is too strong, especially on a light-colored background. Why would I enter a password? And what's a custom tag? You need some more detailed explanations if the users wants it. Your login/register boxes should match the colors of the ones found on the front page for consistency. A quick login box on the front page would be nice.
I like the simple and clear layout, I would just make the logo and the site subtitle "create short url) larger.