From 6th Ed Impressions, Part Ten - Instrumental Changes:
Vehicle overwatch - Regular vehicles can't fire overwatch. If they're having overwatch, I think it would have been cool to carry over a defensive weapons rule, and clarify certain weapons as qualifying, letting them fire overwatch. Things like pintle-mounted storm/combi-bolters, the shuricats on eldar vehicles and other weak, maneuverable weapons. It wouldn't help much, but it would be cool, help keep the vehicle player interested, and more reason for people to put those cool heavy stubbers on tanks.I would take this and clarify it a little bit. Defensive weapons would of course have to be able to target the unit in their arc of fire at the beginning of the Charge. It'd really only make people consider what angle they decide to engage your vehicle from.