Pretty late to the party, but if recent DLC rumors have your interest up, No Mutants Allowed has published its take on Fallout: New Vegas' first DLC.
Dead Money introduces two new enemy types that will stand in the player's path and progress during the Dead Money campaign;
The mysterious Ghost People, strange beings trapped in hazardous environment suits, that may be the descendants of some of the Pre War workers who were employed at the Sierra Madre during the time the bombs fell, and the Sierra Madre's own security system; holographic guards.
The first several times the player engages the Ghost People they can not be killed in the conventional way, just because the player has drained their health and thrown them on the ground does not mean that the Ghost People are dead. Given time they will regenerate and get back up to continue their assault. The only way to permanently kill them is to decapitate them or cut/shoot heir head, arm or leg off. After the player has met Dog/God he can teach the player how to kill Ghost People without having to decapitate them.
While most of the Sierra Madre's Villa structures and technologies have decayed over time, the security system is still functions well, built to last much longer than its surroundings. As the Sierra Madre is still on lock down some of the systems identify the player or anyone else it encounters as intruders that need to be dealt with.
Unlike the Ghost People, the holograms can not be destroyed by a direct attack (there are no pulse grenades), being made up of light after all, while they can cause severe damage once they have detected the player. The player needs to change their patrol paths through computer controls or find their emitter and shut it down to permanently disable the hologram.
Outside of these active threats the Sierra Madre Villa and casino also has a large number of other dangers the player needs to be careful for. Various traps that have been set up by the Ghost People or earlier visitors to the Sierra Madre who wanted to take out their competitors but which sometimes also ended up killing the people who placed them. The damaged intercom and communication grid as well as partly working radios are also a threat to the player and his/her companions, these systems in general have ceased working properly but still transmit waste noise on frequencies that interfere with the bomb collars the player and the companions have been forced to wear by Elder Elijah. Getting too close to a working speaker or radio can trigger the bomb prematurely and the player only has a small window of time in order to find the speaker or radio and shut it off or destroy it before the bomb detonates.