Sexual lust. To some in our culture, it's no big deal. They see lust as completely natural, and even something to be done openly.
But to Jesus, sexual lust negatively affects our relationship with God and with others. This is why the "meek and mild Jesus" talks rather graphically and violently about the issue of lust. But his starting words aren't just to shock his listeners, rather they lead to newfound freedom, so rather than just lust after someone's body, we can actually love them like Jesus loves them.