Attack Rating, Defense Rating, Magic Rating (this includes specialized Magic Ratings) and Magical Resist are the "big 4" stats in terms of comparing attackers to defenders in the game.

The opposing stats are, as may be obvious:

Attack Rating vs. Defense Rating
Magic Rating (+Specialized Rating) vs. Magical Resist

In normal combat, if the attacker's offensive rating is lower than the defender's defensive rating, the damage will be reduced. Likewise, if the attacker's offensive rating is higher than the defender's defensive rating, the damage will be increased.

When attack and defense ratings are equal, the damage dealt will be the "normal" value of the attack, spell, or ability.

Heroic rating works mostly the same; however it is limited in when it can apply. It only applies if that rating is lower than an opposing rating.

For example:

Player A has 250 Attack Rating, NPC B has 300 Defense Rating, outcome is -50 for the Player
Player A has 250 Attack Rating + 100 Heroic Attack Rating, NPC B has 300 Defense Rating, outcome is even
Player A has 250 Attack Rating + 100 Heroic Attack Rating, NPC B has 400 Defense Rating, outcome is -50 for the Player
Player A has 250 Attack Rating + 100 Heroic Attack Rating, NPC B has 200 Defense Rating, outcome is +50 for the Player

For specialized magic ratings, as with Demonologists and Necromancers, the specialized rating is added when using a spell of the school that one has specialized rating for. For instance, if one has 100 Magic Rating and 50 Electrical Magic Rating, the effective magic rating is 150 with Electrical spells and 100 with non-Electrical spells. (The Archmage feat in the Mage general tree will make the buffs apply to both schools for the class, so all spells will effectively be cast with 150 Magic Rating.)

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The last point is simply, these 4 stats do not apply in PvP. Thus, in PvP positive or negative damage multipliers normally associated with offensive and defensive rating comparisons are completely ignored. This is to allow players of all levels to compete on a fairly level playing-field, and not have them dealing artificially reduced or increased damage due to rating differences.

All other stats work normally in PvP. It is simply that "scaler" functionality which is normally in place to dynamically adjust gameplay based on level differences (for instance, the fact that you will evade more against a lower-level NPC) is removed in PvP to allow for a fairer matchup of player skill and ability.

To answer the questions of "why heavy armor?" or others along that same line: all players of all classes have the ability to reach the "target" ratings for a given level. They are a way to control your "virtual level" when compared against NPCs, not to provide a specific advantage to certain classes. All classes should be able to reach, for instance, 317 in all 4 rating statistics at level 80.

Heavier types of armor have higher innate invulnerability ratings, greater resistance to knockback effects, and dramatically increased health modifications. (Heavy armor, for instance, has over 3 times as much health available compared to medium armor.)

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As I mentioned in another post last week, we will be releasing a bit of a guide to statistics and their general functionality in the near future. We would like to make this and other information a bit more easily accessible than it is currently. Obviously, we will not go in too much detail about advanced parts of the system (for instance, the non-linear formula which controls the actual offensive vs. defensive rating multipliers) but we would like to see players having a greater understanding of how stats are intended to work.

Hopefully this was helpful.