Lethality is lessened in low-level 4E the same way as for high-level characters in old D&D: by the ratio of hit points to damage. In old D&D, a typical hit does one die of damage and a character has (up to "name" level) as many dice of hit points as its level. 1d6 damage vs. 1d6 HP is 58.333% chance of a kill. In 4E, even 3d6+4 (e.g., a rogue "sneak attack") is 0% versus 23 HP.
An ancient red dragon's "immolate foe" power does only an average of 32 initially, +15 ongoing (save ends). (The dragon has 1,390 HP!) A fresh 1st-level fighter with 17 constitution (32 HP, 16 bloodied) survives the average immolation if (55% chance) the ongoing damage applies but once, and someone applies first aid or healing magic (or the encounter ends, allowing a "rest") before failure of 3 death saving throws.
In 1st ed. AD&D, the dragon does a minimum of 36 points and even an Unearthed Arcana barbarian has a maximum of 40 -- versus 24 for a ranger, or 14 for a fighter or paladin.
It's a bit harder to get killed in one hit in low-level 4E, unless it's "friendly fire" (PCs can pack a wallop!). Not knowing when to retreat can easily lead to a TPK, but that's a different matter. I think it might actually get more dangerous in the paragon and/or epic tier, but I'm not sure.