You can come up with all kinds of crazy justifications for it to try and make it fit into the lore, but it's really just a game mechanic. WoW would be really, really lame if we couldn't rez, either through corpse runs or players ressurrecting us. It's purely, 100% a game mechanic. The power to resurrect people might be there in the lore, but it's certainly not as commonplace as it is in the game.
If you need to try to justify it in RP or something along those lines, an explanation I see used often is, "It wasn't our time yet." We're still needed for something, we haven't fulfilled our role in things yet. Everyone who has died has fulfilled their purpose. I personally loathe that explanation, but it's the most common.
Rezzing players is a game mechanic, but if you need to explain it away for some reason, just be creative. There is no explanation for it in canon lore. Just wing it. If you don't need to explain it away, then we just don't worry about it.
"Whatever happened to Arator the Redeemer? Is he forever doomed to wander around Honor Hold asking questions about his parents? Kinda disappointing that Blizzard never went anywhere with that storyline as the expansion progressed. Have they ever hinted about doing something more with him or Turalyon and Alleria?"
From what I understand, this was a plotline they intended to work on within The Burning Crusade. In an early description of BC, I seem to recall that they had intended for us to go beyond even Outland. The plan was to go to other Legion worlds besides Draenor, if only for a very brief jaunt. Turalyon and Alleria were supposed to be on one of those worlds, having gone through/been sucked through another Legion portal. This never happened, probably because they wanted to focus on just Outland and its story and not send us to all sorts of other crazy places.
Since it never happened, they're still missing for awhile and Arator is going to be walking around Honor Hold for at least another couple of years, if not longer. And since it didn't happen, that's not necessarily the plan for them anymore. They might show up somewhere else. The original idea for Darion Mograine (being in Outland) didn't happen in The Burning Crusade, that changed completely for Wrath of the Lich King. If/when they show up again, it'll be tailored to that expansion.
"Now that we have officially killed Malygos, isn't it possible/probable that the Lich King might try to resurrect him as a Frostwyrm? What about the other powerful beings that have been killed in WoW such as Illidan and Kael'Thas? Why does he simply not bring those guy back as well?"
First, he would have to be able to reach those people. The Lich King doesn't exactly have a presence in places where he can easily turn Illidan and Kael'thas into the Undead. Outland? The Scourge doesn't do Outland. Sunwell Plateau? Sure, he got in there once, but the world probably isn't going to let that happen again, especially since we're attacking the seat of his power directly. Yeah, he probably could raise those people, but he's not really in a position to do that.
As for Malygos? Good luck assaulting Coldarra and busting into the Eye of Eternity to find his corpse that was zipped off into the void. That's not gonna happen.
"I've been thinking about this question lately. What exactly is Azeroth? Is it a spherical planet or is the world of Azeroth flat? If it was a sphere then you'd think you could travel to Kalimdor from the Eastern Kingdoms going right.
I don't get what Azeroth really is..."
Once upon a time it was described as a flat plane, kind of like Earth once was. Earth was flat for a really long time, too! But no, Azeroth is a spherical planet up in the sky, just like our planet. You can see it hanging in the sky from inside Black Temple, and you see globes of it scattered throughout Halls of Stone, Halls of Lightning, Ulduar, etc.
You'd think we could travel to Kalimdor from the Eastern Kingdoms by sailing East, but do we have a reason for doing that? Maybe it's a longer trip. Maybe there are sea monsters there. Maybe those waters are uncharted, and it's just easier to sail the other way as long as you give the Maelstrom a wide berth. We could travel that way, but that doesn't mean we should or would.