Some of the best (worst?) roasts are the ones that hit too close to home. Whenever someone can accurately destroy you with a specific insult that hurts you on a shallow level, they won.
It's usually something that other people notice immediately, but are too polite to comment on. They take that weakness and then carve a heck of a roast out of it. And ever since the r/roastme subreddit debuted, tons of people have tossed their pictures up online in the hopes that they'd get specifically insulted.
Now not everyone can roast like a professional but the beautiful thing about a community like Reddit with such a large user base is that coming across the perfect roast isn't as difficult as you'd think.