Pure head canon at it's finest, ladies and gentlemen.Pandaren disagree with you.
I think you forget that the Alliance lost a lot of unique features to the Horde when Blood Elves were added to the Horde in TBC. It was the most faction blurring action of Blizzard to date but apparently if it benefits the Horde you don't care about that.
Blood Elf Paladins are direct copies of Human Paladins. The Blood Elf Paladin mount is/was a recolor of the Human and Dwarf Paladin mounts (making them no longer unique). Also, with the way the Blood Elf Paladins were implemented the Horde got a light oriented race which until that time was exclusive to the Alliance. Metzen himself has said that giving Blood Elves paladins broke the existing lore.
The Alliance may have gotten Shaman in return, but they didn't get a shamanic race like the Orcs, Trolls or Tauren. Draenei are a Light oriented race with Vindicators (aka Paladins) as their token class. The most notable Alliance shaman is not even from a playable Alliance race but a Broken (Nobundo).
Until TBC, 75% of the Alliance races had a human colored skin-tone. In Classic the Alliance was basically defined as tall, small and tiny humans plus elves. Blood/High Elf models, although based on earlier lore, are essentially mix of these originally classic WoW Alliance core-race features and as such giving them to the Horde reduced faction uniqueness at the expense of the Alliance.
It's also worth noting that High Elf NPCs in vanilla were already friendly/allied to the Alliance while Blood Elves were hostile to both Horde and Alliance.
I can understand that Blizzard prefers different looking races in Horde and Alliance and that the art department prefers designing "new" races, but if they really cared about faction identity and uniqueness they would have designed a new "pretty" race for the Horde in TBC instead giving them Blood Elves.
Arguing that Alliance High Elves should not become playable because they would blur faction identity is therefore hypocritical since Blood Elves blurred those faction lines already when introduced in TBC.
Go play WCIII first before trying to cry that blood elfs took unique features of the Alliance.