Dolan, why 10 kingdom cards? Did you ever try 12 cards available at a time, or eight? (Different cards, not cards per pile)
As you add more cards you get more options but the game is harder to play, especially when you're new. Of course at a certain point you aren't increasing options much anymore because cards displace other cards for you.
Originally I had ten cards and decided to just put them all out. We could cope with ten so it stayed ten.
At one point I played with eight for a while. It worked fine but was not as good. I never really considered twelve because ten is already too many to remember them all (and I didn't consider odd numbers). There's the neat trick of, it's my first game ever, man I'm not reading all these, I have $4, what costs $4, I'll read those. But ten cards is still a lot.
Another thing is that the number has an effect on the variety you get. With 25 cards and 10 at a time, it takes you, you know, 2.5 games to see them all. If it were 8 cards at a time it would take 3 games. That was the big reason to test 8 for me, and it pushes away from putting more cards out at once.