I can play a 10/3 with no problem, yet KJ kills me. Go figure.
That's because you're probably folding 10/3 all the time. And if you do play (in a blind for example), you're probably going to have to hit the hand super hard for you to continue, otherwise you're folding the hand post-flop. Even if you hit, you're most likely folding to any action.
As for KJ, sometimes it's hard to let go of hands that look really good, have some potential to win big. Sometimes, if you've been card dead, KJ in EP looks like a nice hand to play and often overplay. Unlike 10/3, if you hit a K or J on the flop or some sort of broadway draw, it could be much harder to let go of. Or if there is action on a flop, you might be even more inclined to stick around and over play the hand.
I don't think KJ is a terrible hand at all. It still beats a lot of hands PF and coinflips vs a lot of pairs, but it's also a hand that you have to be able to let go if you need. And not over commit yourself too, unless you're short-stack and you're shoving with KJ.