I just killed the Spider Queen on level 2 of Od Nua.  But I did not get experience.  My main character was unconscious.  Does that matter.

I noticed something similar on my last playthrough to when I did not read any exp gain in the logs when I killed certain enemies.

Figured since its a giant spider that the exp gain would be somewhat significant but its was null.

Can anyone explain this?

Yes.

 

Technically, fights never give experience in this game (a point that was much debated on these forums)

 

When you kill a monster, you get some info for your beastiary, and that "info" gives you a little experience. However, once your bestiary is complete for a certain type (say, spiders), you no longer get any experience for killing them.