This page will serve to explain some of the stats that people seem to be confused about or have misconceptions of.
Piercing is one of the two best stats that you can get on your equipment. The way it works is that it will allow your attacks to penetrate through a percentage of the enemy's Defense stat. This number is usually very high, which is why even though Piercing comes at small percentages, it has a huge effect on your DPS. It is recommended that you want 20-30% Piercing.
Physical and Magic Piercing are stats that will allow your attacks to penetrate through the enemy's Physical/Magic Resistance. The Physical/Magic Resistance numbers are usually way less than Defense, which is why even though Physical/Magic Piercing usually comes with huge percentages, it may not necessarily have more of an effect on your damage than Piercing. It is recommended that you only need 10% Physical/Magic Piercing.
Accuracy is a stat that is currently overlooked by the community. It is recommended that you get around 90-100 accuracy.
Weapon Attack/Physical Attack/Magic Attack
These 3 seem to have the biggest misconception. Weapon Attack is usually a huge number (in the thousands) while Physical/Magic Attack is a small number (in the hundreds). This seems to have resulted in the idea that percentage increases in your Physical/Magic Attack have less of an effect on your damage than percentage increases in your Weapon Attack. However, Weapon Attack and Physical/Magic Attack are multiplied with each other in the calculation of your damage, meaning that 10% Weapon Attack increase does the exact same thing for you as 10% Physical/Magic Attack. This is why even though Balrog's debuff reduces your Physical/Magic Attack by 40% and those numbers are usually only in the hundreds, you will feel like you are doing 40% less damage overall.