When a party is first or second level, it’s just too easy to kill the poor saps. A game can only take so much fudging of dice rolls before it shows through and the players’ victories feel faked.

Around when party members get access to Greater Teleport and Disintegrate, things get a little crazy in the other direction. In order to put a 16th level party in a situation in which they are really being challenged, it needs to survive for a couple of combat rounds against characters that can deal a rather nutty amount of damage in a round. It needs to be able to faze a Fighter with over 150 hit points but not hit so hard that it’ll paste the party Wizard in one shot. It needs to be able to weather some rather spectacular magic, meaning either excellent saving throws or spell resistance. Spell resistance is a game mechanism I don’t really care for, as it results in the DM saying “ha ha, you wasted a spell” a lot. It’s all or nothing and that rather blows. Such challenges also are hard to make plausible within any game world that a 1st level Commoner could get by in.