As some of you may know, a fellow blogger named Fritz has proposed the philosophy of "living in the future" when it comes to playing 40k. He proposes that one should discount any non-fifth edition codices when you are planning how you will build and play your armies, based on the idea of a trend in fifth edition codices to do one thing or another.
I may be biased, but I think Fritz is on to something. The fifth edition codices do tend to have some things in common, generally lower points costs, improved psychic powers and at least one shiny new "must take" unit for instance. As I am a follower of the "take all comers" school, The idea of thinking in generalities is not entirely forgien, but I too get caught up on particular codices and my difficulties against them.
What I'm trying to get at is answering the question "how do you plan for all armies?" Well, the answer can only partly be found in your opponent's new "overpowered" codices, mostly it can be found in your own. Sun Tzu said "He who knows when he can fight and when he cannot, will be victorious. " I know this is only a game, but the principle applies. What you need to ask yourself is "how can my army fight"? Is your army mobile or is it static? Can it fight at range or up close? If you know these answers, you are half way to solving the problem; you when you can fight.
Knowing when you cannot fight is not always as simple, especially when playing against newer codices. The horror squad that was nob bikers can still chew on most enemies, but should they charge a swarmlord with guards or a trygon? There are situations when Tau will be out shot and orks out assaulted, and we need to deal with that reality. But how can we deal with being forced into a situation we do not want to be in? Each army has its own answers built in and it would take too long to detail them all here, but let me leave you with this last thought.
Your codex is like a surfboard you use to combat the other codices. It may be unbalanced, it may not be exactly up to par, but it's still a surfboard. Polish your surfboard and reread your codex. There are little knots in the wood, units you never thought were worth taking. Maybe they aren't imperfections, but a part of the board you haven't yet gotten the hang of. With the right know how and an adaptable mindset you can get on a new part of your surfboard and ride the wave of fifth edition right on the crest.