The Intro scene mentions that the ritual to summon Lilith requires willing blood. But the adventurers seem fairly unwilling and the priest is implied to have been mind controlled. So how come the ritual works? They probably sealed their fates by opening the door in the first place. That blood being offered willingly allowed the rest of the ritual to go through even when they regretted it later.
Symon likely gave up hope when he realised that Elias (as the one-eyed swordsman) had played him for a fool. In-game, the Wanderer can investigate how Symon ended up with Elias and Symon's letters made it clear that he was fond of Elias.
The fact that you can inflict Damage Over Time effects on inanimate objects such as barricades makes a surprising amount of sense - poison is corrosive. Fire likewise also damages these objects. Bleed? you're just causing them to fall to pieces. It also makes sense why you also are able to do this to ice - because you're making it melt.
The Diablo II Paladin, Carthas, died after sealing himself and his men to Sankekur's tomb to make sure they don't harm any others beyond themselves. It's counted as a Heroic Sacrifice. Recall again, what's one of the first skills that a Diablo II Paladin can learn? Sacrifice.
The Time Skip of 50 years between III and IV is to answer why the heroes from III didn't show up to wipe the floor with Lilith. They probably died of old age or are too old to fight her.
All right, so... Tyrael, one of the only good angels in the series, is Put on a Bus, disappearing and never be heard of again. If we have seen the track record of a 'vanishing angel' like Malthael and how the series has gone back to its darker roots... If there is a chance of Tyrael returning, it may be presented as not a very good thing...
Not only that, having him around may have reigned Inarius in and saved countless lives. What could possibly cause him, the Big Good that always meddled in human affairs for the past three main games, to disappear from the plot?
Lorath mentions that Tyrael left because something had spooked him. What exactly could scare him?
