I really liked the idea of the MACOs, who could be assigned to a starship whenever military force was necessary. I hope a future Trek series will employ some form of this plot device as well, as it helps maintain Starfleet's "primarily exploration" theme while allowing for an occasional shoot-em-up.
Of course, the MACO's are yet another device borrowed from Yamato. See season 2 and the addition of Sgt. Knox (american version) and the "Space Marines". They don't quite fit with the crew at first but prove to have enormous strategic value as the story unfolds.