The issue is that by making your long URLs short, they also obscure them, making it harder for you to determine whether it is a good idea to visit.

In my ideal world, the URL shortener would not block you from visiting the site, but would display an interstitial page warning you of the possible problems if you did - much like Google Safe Browsing.

That is exactly the path I took with http://safe.mn/ If the destination is deemed unsafe, you get a warning explaining what is possible wrong (malware, virus, adult content., etc.) + a screen shot (hidden by default) + a link to continue to the site anyway.