Okay, I have heard some people complain about Lydia (and other followers) constantly dying unless they dismiss them. However, I always thought followers were immortal unless I accidentally hit them. Could people please explain?
A lot of people are really careless.
Also, unless I started imagining things -which I find highly unlikely:
- Followers almost always follow the player's exact route -when in relatively close range at least.
- In some occasions, if the player jumps down and the follower doesn't find an alternate route, he'll jump down too -if the height is not deadly.
- If you avoid a trap, the follower will do so too. Don't expect them to jump over traps though -it's not going to happen

No, you`re not imagining things, Lydia or any Follower will jump down with you sometimes. But you have to be really close to the edge and not move off too far after you jump down.
Followers do always follow the exact same route. Also if you get too far ahead they speed up to catch up. If you suddenly stop they can`t stop due to momentun so will bump right into you. You can have fun with this by running then suddenly strafing left or right and stopping and watch Lydia ZOOM right past you again and again.

I`ve seen Followers avoid traps too, but if you get too far ahead, they`ll just run over them and yea, they won`t jump over them - ever.