|Remember GDS stands for Game Design Series!|
These three things are:
Overall Design is how well the book translates its fluff, lore and background into the gaming world. I think this is pretty straight forward as you can pretty much read every unit entry, and then read the rules. Does it reflect well? Does it make sense? This should be well-reflected in the "actual" feel of the codex and its rules. It must feel right to the player playing it, both in consistency terms and in terms of imaginative immersion.
e.g. The Void Raven Bomber is designed to be a more destructive air-ground aircraft compared to the Razorwing. Both aircraft are designed for ground annihilating ground targets, but the Void Raven has more armor, a Void Mine, stronger Lances, and deadlier missile options. Because of its heavier payload, its points cost is noticeably higher than the Razorwing.
Internal Balance is a balancing factor with all things considered within your own army book: How the book comes together, where the units fit in, how do they scale, do they conflict on the force organization, how they're priced compared to other options..etc. Internal is exactly that, it's your book and your book alone. This plays a heavy part in whether or not a book is considered fun or not. Does the book have enough viable options for different army builds? Do those army builds appeal to the new, casual and competitive player?
e.g. Wyches and Warriors fulfill different roles, but are both great Troop choices. Both excel at their respected roles of close combat and shooting, and both are priced competitively. As an Dark Eldar player, both are excellent and viable choices when designing their army.
External Balance is a balancing factor when compared to the game as a whole; more importantly, against other "factions" that exist in the same game. How do they balance out compared to those units in respects to unit cost, function, special rules, force organization, ..etc, is strictly comparing one unit from one army to another. This plays a heavy part in the metagame, general longevity of the army book, and how much fun a player can have before realizing he's either outclassed or is facerolling people.
e.g. Grey Hunters are more cost-effective than Tactical Marines from the Space Marine codex. For less points, you get Counter-Attack, a close combat weapon, cheaper weapon options, and better upgrades. No heavy weapon options, but don't get me started on Long Fangs!
From a player/gamer's perspective, there's no greater importance than these three things for me. The book has to make sense, the fluff has to play out accurately on the table, and the balance must be good. There is no order of importance because all three are vital to the success of the army book. Poor design leads to a lot of raised eyebrows and that's always no good. Shoddy internal balance means less player options, unneeded restrictions and general frustration. And well, bad external balance means you're either too weak or too powerful to compete fairly in the current metagame with the rest of your peers. No one likes that.
Let me know what you guys think about this!