How can things be bad examples when they are re-used? And yes, everything you mentioned, all of em had "a reason" or "a way" to return to azeroth once more.. But the fact still remains, it's blizzard that comes up with something and bakes it into the lore somehow to resurrect them (which was the answer to trassk's question)Kael'thas, Cenarius, and DW children aside, your examples are kinda bad. Well of Eternity happened 10,000 years in the past, so Illlidan was more than alive back then, he has not been resurrected after dying in BT. Anub'arak was technically undead to begin with at the start of WotLK, and it's not like it's hard to re-raise him (though I agree they could have found either a better initial use for him like having him in spider wing over that other random spider lord or to have picked a better boss for the end of ToTC). Ragnaros, already in lore that elementals can't be killed unless they're in their home plane, so by finishing him in Firelands he should be "dead dead" now, and thus could have been re-summoned otherwise like DW did to bring him to Kalimdor.
Have a lovely rainy day!