What is the appropriate reaction when you encounter a snake in the wild?

It really depends on where you live. In Michigan, where I live, we only have one type of venomous snake that is somewhat common. Therefore, it is generally safe to approach a snake even if you aren't a snake expert or can't ID every species.
In other places with more venomous species, however, caution is required if you don't know your snakes. A good rule of thumb is that you should always assume it is venomous for your own safety. This does not mean you need to be afraid of the snake, but give it plenty of room to avoid you (and for you to avoid it, as many snakes can strike 2/3rds of their body length).
As was mentioned, most snakes are very non-aggressive. The few that are don't need to be feared so much as avoided/respected in their habitat. A few species may actually chase you; run, don't kill the snake. Never kill a snake for biting or attacking you. 99% of all bites are human-caused; you pick up a snake in its habitat and it may try to defend itself.
In the end: if you don't know, don't touch it. But you don't need to be paranoid, as many can be observed from a safe distance regardless.