I think the issue of challenge levels for parties and lethality of encounters is sort of the sandboxes problem of evil. It is the one big question that can really undermine the validity of a sandbox. It seems to get at the heart of some core concerns, so It think it is definitely worth discussion and thought. My feeling on this is I don't want the world to have a sense that it is oriented towards the PCs power levels, but I also need to have a sense of fairness in how I manage these things. If a GM wants they could have 30 dragons swoop from the sky and incinerate the players. So if the GM really hits the gas, the party can be killed. But I also want the possibility of facing overpowering foes, being killed or experiencing a total party kill. For me being balanced about it, using things like encounter tables, giving thought to why a particular result is there and really trying to be fair about it, and giving the players a fair chance. One important thing to keep in mind is a lot of threats, unless their goal is to eat the characters (which it could be) don't necessarily want the characters dead. Some might just want them out of the way, might want to take their things, or might want to coerce them into leaving an area. But character death is very much on the table in my games. Usually when something more powerful shows up, I consider that a moment for me to be very conscious about how I am GMing, and make sure I am giving players the full array of options they ought to have in that scenario (i.e. if it was a big lumbering dinosaur of a monster, did I give them a fair indication of its approach or allow them to make some kind of survival or detect roll?)