Assassins are not feat starved anymore, so what else is really worth taking? Arguably, the melee power from weapon focus is of greater benefit to assassins than any other class/build due to the fact that sneak attacks are affected by 1.5x melee power. It's still a small bonus but, again, what else is there? Weapon finesse is good while leveling, but not worth it at endgame imo (due to not needing to swap Thunder-Forged weapons because of their extreme superiority over nearly everything else). Toughness is also a small bonus and I'd prefer the extra melee dps than a negligible amount of HP.
What would you consider taking instead of weapon focus pierce?