One of the hallmarks of the very best players in 40k is their ability to bring an army that looks like a random sample of units from a codex, and continually trounce upon opponents of all skill levels. I don't consider myself quite that good at the moment, but I'd like to think I'm approaching that direction, especially with my primary army: Dark Angels. This actually isn't something that every codex is capable of, at least not in the most competitive scenes. Sure, great generals can take weak codices and do some significant things, but it's almost always due to vast amounts of player skill and an understanding of the army's strengths and weaknesses. Armies with better tools, more efficient tools, and multiple units with similar roles are much more able to pull this off. Right now, I'd probably limit just a few codices in the game to pulling this off really well. Space Marines, Necrons, Imperial Guard, and Dark Angels are the best, with Chaos Space Marines and Space Wolves coming in just behind them (in my opinion). Other codices are extremely competitive, but have very few competitive options.
So what's required to make and play one of these amazing "battleforce" armies? Let's take a look: