Interesting results. I think this is a really interesting dominion variant, but I doubt it will have the effect of just allowing people to play auto-match with !possession, !saboteur or something. Having 20 cards, vetoing and choosing a random 10 of the remaining ones would mimic that much better. When you see 12 cards, and know basically exactly how the kingdom is going to turn out, you can veto much more strategically, and it ends up being part of the game rather that just a pre-game deck selection modification.
I see 4 classes of cards getting vetoed (assuming strategic vetoing):
1. If you think you're better than your opponent, you veto cards that increase variance like tournament, treasure map, and possession.
2. If you see a strategy you like but that would be stopped by a particular kind of attack, you veto the attack.
3. If you are not confident in your ability to compete in an major engine-building game and there is only 1 village or 1 +buy card, you just veto that single card and force a low-action count "big money" game.
4. If you're not good at colony games, veto a prosperity card.
The one I'm worried about is 3. Out of 12 cards, a single village or +buy card can be a pretty common occurrence, so it may be possible for people to force a lot of less interesting games. However, from the results, it appears that is not happening so far. So we'll see