It's kind of embarassing... but I'll tell you so that you hopefully won't make a similar mistake. Here's a rather detailed explanation of why, and what I am doing differently this time.
I had to remake my pally at level 89
, because his life was way too low. It was at max 3.4k or something like that, and that was with starstone and moonstone rubies in his armor and helm. I put way too many points into strength and dex, worried about not being able to wear heavier gear, achieve max block, or even have a decent AR, and as such lost a lot of vitality points.
There are many people here who say leave strength and dex near base. I would say raise them to no higher than 50 or 60 for each, put no more than 10 in energy, and everything else in vitality. That way, you shouldn't have too bad of a time wearing decent armor until you can get
Even with close to 50% DR-- 30% from shaftstop, 10% from Rockstopper, (amazing low-level helm), 19% from a near perfect Knight's Vigil and an optional 14% from String of Ears-- I was still taking unacceptable amounts of damage.
The Reanimated Horde could knock my life down from a couple thousand to a few hundred too quickly, and when I was amped by Succubi in the dungeons leading to Nihlathak, things became very perilous. On normal, they aren't too bad, but by nightmare, their Charge skill level is 5 levels above what it was in normal, and possibly more for unique and champion mobs.
I guess that through charms, amethysts in helms, +strength items, and Hel runes you could get enough strength to wear Tyrael's. And even if you don't get 232 strength, a lot of high level unique armor and weapons have reduced requirements, or are socketed so that you can reduce them.
More life is better than having 50% damage reduction, because something capable of inflicting a 4000 damage hit will still hurt you very badly even with 50% DR, if you only have around 3.4k life.