Usability: Make the URL textbox bigger and autofocus it on load. Give the OGNX labels a bit of rounding to make them look like buttons. Give them some highlighting when selected. This will keep people who don't know better from getting annoyed with those little circle toggles. Submit goes on the bottom-right. s/custom alias/alias, add a hover or question mark to it that explains what it's for. Add some overlay js/ajax to give me the new link without having to go to a new page.
Design: Align your header's (logo, speech bubble) width with the google-advertisement-banner-footer width to make the middle stand out. What's up with affiliates? Put that junk into the box right under the banner. Your copy/link/share link colors are ugly. 2 lines of text max in your description.
Copy: Comma or exclamation after 'Congratulations', period after 'others'. Some of your wording is unprofessional. Don't try to be funny. Some bosses won't look kindly on a site like this just because of your language. For example, your http://isnsfw.com/44dfs6 'porn star' might get hit by someone's work filters and get your site blacklisted.
Good plan. Go find some decent sfw url shortener sites, see if you can set up mutual linking with them ("Have something sfw to share? Try [random friend]"). And do you realize that your source of revenue is under the fold on 900px height netbook screens? Many college students use those.
(When submitting, put your comments into the the actual post, not into a comment. Title should also be 'ask hn, feedback on nsfw-friendly link shortener isnsfw.com')