• A scimitar is a slashing weapon that deals 1d6 damage and deals double damage on a critical hit but has a threat range of 18 to 20.
• A vorpal weapon is a a magical slashing weapon that decapitates on a critical hit. The "vorpal" ability is equivalent to a +5 bonus on a weapon.
• A keen weapon is a magical slashing or piercing weapon (according to errata) with a doubled threat range. The "keen" ability is considered equivalent to a +1 bonus.
• A weapon can have up to +10 in abilities and bonuses, so long as it doesn't have more than a +5 bonus.

Oh well, nobody's perfect, game designers included...