The only actual deaths I've seen so far, running both LFR and home games since day 1 (minus a few months last year), were in one session of LFR. The party was mostly level 1 running 3-4 bracket (by request), and one of the encounters was with level 5-6 creatures, including a leader with "badass AOE when bloodied" (the AOE took out most of the party, and the survivors finished off the remaining PC with a lucky crit on a high damage melee hit). Even then, it wasn't a TPK because the remaining few monsters fled (per module instructions), and only 2 of the 4 PCs actually bled out.
Only other time I came close was running a slightly modified beholder in a paragon game. Some of the PCs had the petrification ticker on them when the beholder went down, and wound up getting stoned because I made them continue with saves after the encounter ended... (but the survivors just teleported back to town and got them fixed up). I say "modified" because I ran the fight with completely randomized eye beams (since if you run the creature as written, the creature can choose to completely shut down a given PC with the sleep ray, which leads to the trivial "sleep the healer, disintegrate the tank" strategy).
In general, I'd say the threat-level situation is "working as intended" and a result of certain design decisions (e.g. PC death is bad because "it leaves people sitting at the table with nothing to do", one-shot kills are bad, the fights have to be long enough for "tactics to develop", etc.). I agree that lower-HP/higher-damage monsters help the situation somewhat, but I disagree that three-saves-to-bleed-out is substantially different from the negative-10-death's-door rules. (I don't have monsters coup-de-grace PCs that are already supposedly "mortally wounded", though, so that may be why.)