His ads. Specifically, his ads on individual blog posts on Boing Boing[1. Also: what the hell, Boing Boing?] that start playing an audio track that you can only shut off by clicking on it. I don't want to click on Romney ads. I don't want to hunt through my open tabs to find the one where an ad just started playing. And I will not support a candidate whose comprehension of the basic principles of web communities is so poor that he think, or his campaign thinks, that it's okay to create ads that invasive.
There are a lot of broad principles flavored reasons I don't support Romney. But the terrible decisions conservative campaigns so frequently make also speak to their basic lack of interest in the quality of life of the people who vote for them. Romney's ads aren't just dishonest, needlessly combative, and emotionally exploitative. They're also annoying, and they make the parts of the internet on which they show up a worse place.