Realm 2 - logged in at like 7-8 server time.
Undead starting area was just about perfect. Mobs were scarce but quests were doable, just took a little longer. After getting through the starting area, youd still see a decent amount of other players, but it felt a lot emptier.
Tauren starting area was a lot less populated than the undead. After clearing the starting zone you would see almost no one while questing.
Compared to an expected launch day, it was less crowded than expected.