1. They are Sunwalkers. Different"light lore-wise. Human paladin light is holy. Nightelf get their power from the Moon and Tauren from it's opposite the Sun. Two sides to a coin that is The Earth Mother. It would be just weird to have identical spells with different names for Tauren.
2. Night elves were always capable of using magic. It was just taboo. After the Sundering, night elven society outlawed arcane magic on pain of death. In Cata we saw a Highborne agent sought an audience with Tyrande Whisperwind, to offer that they put their differences aside and combine their resources and skills to prepare for deathwing
3. Well they are not human. The first gnomes that came into existence were mechagnomes who were turned into gnomes with the Curse of Flesh...didn't say anything about making them weaker...just mortal. I get appearances maybe weird but that does not make them weak.
4. Whats wrong with that? If we can learn from them why can't they learn from us?
5. Undead DK's is not an undead that has been reanimated. It is Scourge that was once freed by Sylvanas that became the forsaken that were reassimilated / brought back into the fold as scourge under the LK's influence. Undead DK is really a result of a power struggle of control of your will. Will of the LK vs Will of the Forsaken.
See undead intro text
You don't remember me, do you? We were humans once - long, long ago - until Lordaeron fell to the Scourge. Your transformation to a Scourge zombie came shortly after my own. Not long after that, our minds were freed by the Dark Lady.
A pact was made, brother! We vowed vengeance against the Lich King! For what he had done to us! We battled the Scourge as Forsaken, pushing them back into the plaguelands and freeing Tirisfal! You and I were champions of the Forsaken!
Listen to me, <name>. You must fight against the Lich King's control. He is a monster that wants to see this world - our world - in ruin. Don't let him use you to accomplish his goals AGAIN. You were once a hero and you can be again. Fight, damn you! Fight his control!
Honestly the only weird one is Goblin Shaman because they use tech to force the elements vs being attuned to them which is a technicality vs actual lore behind it.