List building. Something that is on many minds, be it Stelek over on YTTH, or any number of others. This is a skill that take time and experience to hone to a fine edge. To me there are three things that go into list building.
Form- First and foremost is the basic limit of "What models do I have?" You really cannot build a playable army of Tyranids if all you own is Legos. Yes, I know that "stand-in" "counts-as" armies exist, but for those to be playable you need to either be play-testing, or someone like Dan over at teninchtemplate (his stuff is awesome). If you only own 1 rhino then running a list with 3 in it isn't that viable, is it? Some folks can go out and drop a paycheck to build anything they damn well please, but in reality most of us have to build our armies piece by part by little dude. So keep in mind when you are making your list just what is available in your box of guys.
Fluff - Oh the bane of WAAC and troublemaker for internet lists everywhere. Some consider this to be a pointless apect of the game. I myself have read ZERO of the novels and find most of the fluff in the codexes to be crap. Others say that this is an all important aspect of the game and thye bend their army to fit fluff standards (Eldar players...I'm talking to you Mr. Lyanden). Whether this makes your list better or not is largely in the mind of the player, as I have seen and played fluffy lists that steamrolled my littlle army.
Function- This is where some hot debate on the internet lives. Is it an "Ard Boyz Take All Comers 2500 point" tournament list or a 500 point Kill Team/Recon Force list? Is it tuned to kill Bob at the shop's list of Thunderwolf Cavalry spam? Whatever you need to do you can do in 40k, you can accomplish with the right list build. But "can it do ______?" is the ultimate question, be is against MEQ or not, Nidzilla or otherwise. My personal preference is a blanaced list that is flexible but not overly spammy. I like to play 2 dreads with twin-link autocannons and psybolts, not "I-Net Lord GrayKniggit"'s ever vaunted trio. Why? Because I like that last slot open, for further experimentation. Possibly slotting in the Land Raider, in whatever configuration, for the mobile bunker and bullet sponge, or even shoe horning in a purgation squad to get a scout move for easy set up from a GM. Also I only own two sets of rifleman dread arms, and I prefer to play WYSIWYG as much as possible.
But over all I don’t want to be “that guy with the purifier spam list” who no one wants to play at the local shop. Already most of the guys don’t want to play my GK even if I was playing them before they even picked up a single box of figures. Why? Because everyone sees the GK right now and shit themselves with fear. Yes, the new GK ‘dex has subtly rewritten some ground level meta concepts of the 40k ideals of list building and play. No they do not warrant any more fear than any other army. A stupid general playing the most bad-ass Space Puppy list of uber –Interwebs-pwning action will still lose to a relatively intelligent Eldar player who is able to rub a couple brain cells together. Trust me. I was a new player, albeit a smart one thanks, playing Eldar. An average of 4 of the 7 regular players who frequented the shop that I played at were Space Solves players. And the one that annoyed everyone, was most often there and hunting for a game when I walked in. He annoyed everyone because he would download and play the latest net lists and get kicked square in the balls with it every-fooking-time. Even by an inexperienced Foot-dar player like me. Yes, my list had some internet built into it but not truck loads. For crying out loud the first “new” models I bought were Swooping Hawks (the most useless little dudes in the game!), they played 2 games before hitting ebay. But even then I did thing my own way and killed him 5 out of 6 times (it is foot-dar, you can only get so much out of them some days). That one win on his part? Thunderwolf cavalry spam fresh from some big tournament winner’s list. Did it eat my guys? Yes? Why? 2 reasons really, he had awesome luck with the psychic hood on Nyjal that day, and overconfidence on my part. Did I throw away the game and “lose”? No, he won fair and square.
In the end my point is no matter how hard you build a list or how fluffy, it will take some skill to use it effectively. I am a big proponent of a thinking general who plays to win, but understands the most important rule. It is a game, Have fun.