keen scimitars and abuse
The real abuse starts when a character takes 7 levels of Weapon Master from Sword and Fist, gaining the Ki Critical. Now they crit on a 10 and Higher:
18-20 base (3 slots)
15-20 with keen (6 slots)
12-20 with improved crit (9 slots)
10-20 with Ki Crit (11 slots)
As others have said, this isn't that bad since it only does 1d6 damage.
However, since you can't have this until 12th level, let's consider a a 15th level character (starting gold 200,000). He'd get a scimitar, +1, sure striking (S&F, +1 equiv., treat as +5 for purposes of damage reduction), keen (+1 equiv.), vorpal (+5 equiv.) for a total of +8, which costs 128,00gp, giving plenty of money for some magic armor and stuff like that.
Now he straight-up kills any humanoid he hits (assuming he needs a 10 or higher to hit). It is seriously whacked out.
That said, if one of my players did this, they'd run into an aweful lot of undead
