Push is a low level skill that forces your foe into the resting/sitting position. This has a few uses (Resting targets suffer a damage modifier, sleeping targets even moreso) but is usually not used very often.

Kidney shot causes -str, and a good bit of damage.

Gouge causes -str and bleed.

Stacking strength penalties imposes sub-penalties on skills reliant on strength (such as parry!), as well as skills that are a based on a strength derivative (Str determines carry weight, a high % carry weight reduces dodge, -str causes 40lbs/100lbs (40%) to jump to 40lbs/20lbs (200%) for example.)

-Str also causes people to be unable to wield their weapons, and fumble them into their inventory.

For these reasons, most people run around with +str (and +dex) gear in varying quantities to mitigate the penalties.

If you're running around with Kidney Shot/Gouge, try picking up a poisoning weapon (poison inflicts even more -str) to help ensure a weapon drop.

Thugs are the most warrior-ish of the thief classes. Like all thieves, however, they are very PK oriented.