Consistent, reliable coin grading is not difficult. (Although nearly everyone else posting message will disagree.) A "grade" is simply a combination of the amount of wear/abrasion on the coin, plus external damage such as bumps, scrapes, scratches and so forth. Both these can be determined using standards accepted long ago, or with degraded versions shown in the ANA Grading guide. (With the exception of Uncirculated coins, where these are not well defined.)
It gets complicated, confusing and largely worthless, when subjective factors such as luster, strike, tarnish, "look" and other uncontrollable conditions get layered on top of, or in between largely objective criteria.
Today's collectors and dealers have largely handed off grading to independent for-profit companies owned by buy-out companies, and thereby given up meaningful control over quality. It is all about the money - nothing else.