Eb told Harry that by being a Starborn he can punch an Outsider in the face, and it would actually hurt. I think this was literaly, and figuratively. His spells can hurt them, but also when he started physically attacking Behind, it seems stunned, and caught off guard.
I think that was only half the truth though. Mab, and Titania are afraid of Dresden. The last time a Starborn was running around both Queen's died. A Starborn was brought in to help the Fomor (I think to hurt Mab, or distract her), AND Drakul, a Starborn is on par with Mab...
I think what this could mean is that it's not just Outsiders that Starborns can hurt, but other immortal type being like Mab. There is a catch though. Harry was able to hurt an Outsider in PT, but said it only seemed to be for a fraction of a second. He has the ability, but not the raw power. That could change though. A powerful Being could grant him power for a favor, or he could do ritual magic like shadow man to turbo charge a spell, or he could tap into a certain ley line under a certain Island. Drakul obviously is already jacked up on power and that's why he can hurt Mab.. Not because of raw power but because of his Starborn status it allows him to bypass much of her defenses.
That's what this fuss is about Dresden. Wizards aren't allowed to get involved with politics because Merlin learned the hard way that too much power corrupts (per woj). Imagine the potential for an impulsive, hot head like Dresden with his back against the wall.... He could have traded to save Maggie to kill a demigod or something. Or create a following. Why not join with someone who can slay an immortal?
This is what I think is going on. Dresden has the ability, just not the raw power. However if he learned the truth he could figure out how to obtain that power. This is also why Justin, Chauncy, Mab, and others have wanted to control Dresden. They both want and fear what he can do.