The first thing that pops up in my mind when I think about snakes is Orochimaru in his true form. The snakes (summonings) vary greatly in size, body mass and species, from small enough to hide in one's sleeves to larger than buildings, and thin enough to be used to tie someone's hands to strong enough to choke a gigantic animal like the Island Turtle, which was seriously big (as seen recently, and demonstrated by the genetically enhanced clone of Manda). Though snakes are the signature summons of Orochimaru, like toads are Jiraiya's signature summons and slugs are Tsunade's.
But throughout the series we've witnessed a variety of snakes, small, large etc. While most snakes are very obedient and do as they are instructed, the largest snake, Manda, demanded a sacrifice of a hundred humans in exchange for helping Orochimaru. So mostly snakes are very obedient, though there are some exceptions such as Manda, his attitude differs greatly when comparing him to his kind. That also answers that snakes can be very dangerous since Manda even threatened his master.
There isn't much known about snakes in the NV, though all snakes reside in the Ryūchi cave, similar to the Toads in Myobokuzan.
That's all I know ^^