@Nemuraan
High Elves were in Vanilla. They were a re-skinned Night Elf. Blond hair, peach skin night elves (Hince why it was commonly said that Sylvanas used to look like a Night Elf). When Blood Elves got their own model in Burning Crusade, all the High Elves were retroactively changed to the new model.

They kinda looked hideously used to look hideously ugly

http://images4.wikia.nocookie.net/__cb20061206234556/wowwiki/images/8/8f/Gilthares_Firebough_old_model.jpg
http://2.bp.blogspot.com/_Vef63l-sFgE/SLRNrpURlyI/AAAAAAAACp8/9uxOc1P4kZE/s400/SylvanasWindrunner2.jpg

@ash
The main thing separating the High Elves from the Alliance in-game is the Night Elves. As of right now, the Night Elves still view the High Elves as traitors for using arcane magic. The High Elves still aren't really Alliance; they are more of a neutral faction that favors Alliance. Of course, in Cataclysm, Night Elves are going to resume using arcane magic, so that one goes out the window.

I think at this point, the main reason the Blood Elves are staying with the Horde is because the Horde had their back. As petty as elves often are, I think many Blood Elves have grudging gratitude for the help they received during Burning Crusade. Similar to why Taurens are with the Horde.