One of the things that is always popular is talking about a new codex when it comes out; everyone weighs in with their opinions of things, saying which units are bad, which units are good, what the builds are going to look like, etc, etc. Unsurprisingly, it is the opinions of the more-respected members of the community that usually get the most attention, but nearly everyone likes to get their two cents in, and certainly I am no exception to this, as I've been on both sides of that particular fence. Sometimes you're right, sometimes you're wrong, and in the end, only time will tell, but I think it's important to have an understanding of the underlying systems for assessing a new book when it comes out. Too often, the internet gets worked up about some minor point or another of a new codex, completely ignoring other factors that make that irrelevant (e.g. the Mawloc or Dreadknight.)
Read, Read, Then Read Again
You are not going to understand the book on your first readthrough of it, nor on your second. Probably not your third, either. I would not bet good odds on the fifth. In short, you are going to have to read it over and over again if you want to get a good idea of what's going on. This is unavoidable- human beings do not have the capacity to process and correlate that much information in one or two or ten sessions, so just admit it and give it a little time. (Also, that: if you post a review the day after you read the book, it will be wrong and dumb; give yourself some time to look back over and consider things.) it is pretty much inevitable that you will miss details, misread things, assume things that aren't true, and otherwise screw up or overlook important parts of the book; that is why it is good to have people to discuss things with at this juncture, as their insights will be complementary to your own. As to what you're looking for... well, that's up next.
What Can I Do That Is Unique?
Games Workshop, for whatever other problems they've had, have at least made all of the new codices fairly unique. (Umm, except Sisters. Sorry, Sisters.) With each new book, we're introduced to unique mechanics and features- or even just unique combinations of these things- that give us new ways of looking at playing an army. Space Wolves, and later Dark Eldar, brought us Beasts units that could threaten charges across a long distance on the table while still being both resilient and dangerous; Blood Angels brought an entirely new style of army (DoA) that had flexible deployment, good mobility, and terrifying ability to shrug off damage. Tyranids threatened to fill the tabletop with monstrous creatures, souped-up little bugs, or both at once, pushing target saturation to its limits. Each of these books brought something entirely new to the table, something that other armies simply could not do; for some of them, it was born out of their army special rules, but for many others it was simply a feature of the tools the army was provided with. This is the first and easiest thing you will want to be focusing on- unique options in the codex. Can you take transports for mis-sized squads? What weapon options do your units have, and what limitations are there? Do your wargear or special rules do something unusual for other armies, or do they interact in meaningful ways? Look especially to Troop units here, because they will always be the core of an army, even if you aren't spending the bulk of your points on them; what they can and cannot do will dictate which specialists you need to rely on and what your strategy for claiming objectives will be.
What Can I Do More Efficiently?
Sometimes the strengths of a codex are not in something it can do that others cannot, but in simply being able to do something better than anyone else. For example, Long Fangs are hardly the only way to bring 4+ Missile Launchers in a squad, as CSM, SM, and other books could already do this, but they were the first to be able to do so for such a cheap price. Dark Eldar's ability to bring a hail of anti-infantry shots on their transports was, in many ways, similar to Eldar's ability to do the same, but the gap in price makes them into totally different armies. Differences in efficiency are often cited by internet pundits as clear signs of codex creep, but this is rarely the case- rather, they reflect the different strengths and weaknesses of each army and influence what it can bring to the field. It is important, even at this stage, to keep in mind that just because something is cheap, effective, or both does not make it an automatic inclusion in armies. The units that make up an army will come from a combination of what it needs and what it has available; simply taking all of the "super-efficient" units results in disasters like Meltavet or Jokaero spam, neither of which are actually any good because they find themselves unable to fulfill necessary roles in the army.
What Am I Missing?
Just as important as the first two points, especially when evaluating the various Marine variants out there, is to look to what an army doesn't have. Space Wolves don't get Combat Squads- this necessitates bringing a larger concentration of troops than other Marine books; Blood Angels pay more for their tanks- this makes sit-and-shoot units like Tacticals much less attractive, as they waste the potential of Fast. Grey Knights have very little access to Melta and long-range guns, hence the near-mandatory inclusion of Psyflemen and/or Inquisitorial support. Not just presence, but also price and features are important when evaluating units for what is "missing."
What Basic Strategies Do I Employ?
At this point, you should have at least a basic grasp of what the book is capable of and how it will play; for example, determining whether the army is aggressive or defensive in general nature, how it deals with the issue of mobility, the outlines of mechanized, foot, and hybrid armies, how it scores objectives, and what the major weak points of the army are. This, in combination with (and as a result of) the other points, above, will give you a good idea what the general builds and strong points of the army are. And this is also where it gets difficult.
Going through all the above, on some levels, is pretty simple. No one has trouble reading a book (except maybe a guy with vision problems or a toddler.) It's not hard to make a guess at what a "good" price for something is when you have other examples to compare it against, nor to do likewise and see what options you don't get that everyone else does. But collating and corroborating all that information in a useful way is not easy. In fact, it's essentially what this game is about- if it were easy, the game (like Tic-tac-toe) wouldn't really be any fun anymore. So this is where my "instructions" are going to break down, because there is no real system for putting together the actual analysis of the codex and figuring out how to play it; it's something you'll just have to do, over and over again, until you're good at it.
But all of the usual principles apply: seek out information and opinions of other people, and give them due consideration even if they seem absurd, especially early on. Argue about it with other people, and read the arguments others have had. LEARN. I can't overstate this part enough- you simply are not going to come up with a good assessment of the book on your own, no matter how smart you are. Go out and find other people and pick apart what they say for information; you want to be an info vacuum, gathering everything you possibly can. Just as importantly, throw out everything you knew before- what the old codex could or couldn't do is irrelevant now, and if you try using your old knowledge/builds, you are going to find yourself failing.
Evaluating any codex is always an ongoing process, and you will (hopefully) learn more as you write lists and play the game; never be content to fix yourself in an opinion of a unit. Again, this is where seeking out information will help you- I tend to go out of my way to read blog posts about units I don't like (from a gameplay perspective) because I know my bias against them may have caused me to miss things about them. That doesn't mean you need to always take these differing opinions to heart, but you should at least go over their arguments and figure out for yourself why you think they're wrong. As you use the codex, you may find problems you didn't see, uses for units that weren't obvious, etc- certainly I've done an awful lot of this going through the Tyranid codex over the last several years. Keep on testing things out to see what works, and ignore the "sky is falling" chorus that will sound every time a new book is released.
