Besides all your stupid pins and lack of quality descriptions (this one is totally on you, guys!).
The fact that the front page, when I’m not signed in, has a lot of awesome stuff but I’m not sure why it’s there. If I click something and forget I’m logged out, it doesn’t return me to the same page after I do log in. How stupid. Then I have to try to search for the image, which is difficult because.. see above. I don’t even know what the front page is. Is it just random? Can I even return there when I log in?
The second thing is that the website does a horrible job of recognize dupes. If it would just check, based on image URL or website URL, the whole thing would run more smoothly. God, I hope that feature is coming.