These three questions from Brendan's 20 questions deal with what kind of threats the players can expect to face.
9. Will we need to run from some encounters, or will we be able to kill everything?
Yes, but just about anything can be defeated in some way, if not killed.
10. Level-draining monsters: yes or no?
Yes, but recovery is easier.
11. Are there going to be cases where a failed save results in PC death?
Yes, but if you think you can prevent it, try it!
I can't think of any monster that cannot under any circumstances be killed, but I won't rule it out. There will definitely not be much balance in my dungeons, though. I have been keeping the danger pretty low so far, but even that resulted in a TPK last time we played.
So far, level drainers haven't shown up. However, when they do, I will explain that the characters lose levels but not xp, a house rule I've explained previously. Thus, you can regain one level per adventure until you are back to normal.
We've already had one failed poison save resulting in a character nearly dying, but last-minute intervention averted actual death. So death as a consequence of a single failed roll is possible, but it can be circumvented with a clever response.