Counter to the rest of your post, Pete has specifically said that the reason they got rid of classes was becuase they didn't like how you defined your character at the beginning of the game. They want you to just play the game and have that define your character, not the other way around. This mantra goes direcl;y against picking a birthsign at the start of the game. Why would you eliminate classes, but still allow someone to choose a birthsign. It makes no logical sense.
Actually, it makes perfectly good sense. Classes had a big impact on how the whole game played for you, since they affected which skills you needed to use to level up. Birthsigns, by contrast, tended to be much more subtle, giving you a bit of a boost to some starting stats, or a once-a-day special ability, or something like that.
If Bethesda were going to take that mantra you mentioned too seriously, then you might argue that they should also not allow you to pick a race, or make them all the same but for cosmetic differences - after all, if they have differences with respect to how they play, how can you choose between them at the start without knowing how they'll turn out? But we know, from Todd's interviews, that he wants the races to have tangible gameplay differences.
I think Bethesda figure that racial/birthsign differences are small enough to not make such a big difference later in the game, so you won't really feel like you're stuck with a bad decision. But classes can feel like that, so that's what motivated the removal of classes.