Some things in 40k are important, but not quite enough to make an article out of. I figured that having a communal thread where less experienced players can ask questions, and more experience players can give short-form answers - or indeed answer any questions you think should be asked!
I'll periodically update this post if it gets off the ground.
Q: Why should I bring combat units? I thought 7th edition was a shooting edition?
A: It is, but sometimes melee facilitates a much easier counter to something than shooting it. Take Decurion Necron Warriors in good cover - hard as bastardy to kill with shooting, but a charge can deny that cover, and wipe out the whole unit with a Sweeping Advance. Alternatively, you can deal with shooting units by fighting them - Scatter Laser jetbikes have a horrendous damage output... at range. In combat, they're basically Tactical Marines, which means they're pretty dang terrible. You neuter the unit if you can get into combat, let alone win it.
Q (part the deux): What makes a good combat unit?
Being able to reach a fight and being better than what they're fighting. Anything can be a melee unit, and many things you wouldn't expect act as the melee deterrent in a lot of lists. One of the things that I believe distinguishes a good Space Marine player from a great Space Marine player is knowing that against Tau, Imperial Guard, or Eldar, your army is a melee army that can also shoot. Remember, a unit doesn't have to have 4 Str5 AP3 attacks each and a 2+ armour save to be a good melee unit - sometimes, it just has to be able to reliably beat Tactical Marines or Ork Boyz and be able to get into arm's reach. It's fun to have a 10-man squad of Paladins with an Apothecary and Master-Crafted Falchions and Daemonhammers and a Brotherhood Banner and Sanctuary and Hammerhand, but realistically, you're only going to need 5 Paladins in basically any battlefield situation. You don't need to be good in melee to be a good melee unit - you only need to be better than your opponent!
Q: What is offensive and defensive play? Surely you need to get objectives, so how can you play defensively?
A: Offensive play is when you have a plan and you try to enact it. Defensive play is when you counter the plan your opponent is trying to enact. While you can certainly have one of these roles in mind when you build your list, it all depends on what the situation is on the board at any particular time - the defensive player is one who has a beneficial game state (i.e. he's on all the objectives, or is in range with his guns while his opponent is not), while the offensive player needs to change the game state (has no objectives, or is out of range while being pummeled by enemy fire). Say we have two lists - Tau gunline and Drop Pod Space Marines. Who's the offensive player in that matchup? If you said Drop Pod Space Marines - you're wrong! The Space Marines player is going to start off with a bunch of excellent, short-range shooters and resilient Objective Secured units all over his squishy, long-range enemy's backfield. If the game carries on in that way, the Space Marines are going to win. The Tau need to change their plan (sit there and shoot while gradually moving out to get objectives) to something that's going to push back or kill all the Space Marine dudes.
Q: I heard it's better to make a heavily skewed list than a Take All Comers list, is this right?
A: There's a common misconception as to what 'Take All Comers' (TAC) lists are. It's simple, really - a TAC list is exactly that! What a TAC list is not is necessarily balanced within itself. Bringing 3 Venoms full of Trueborn with Blasters, 6 Venoms full of Warriors with Blasters, and 4 Razorwing Jetfighters is a TAC list. It's not balanced, but it can deal with infantry, heavy infantry and monstrous creatures through weight of Poisoned shots; it can deal with light armour through weight of Str8 AP2 shots; it can deal with heavy armour through massed Lance; it brings its own Flyers with anti-tank weapons to counter enemy Flyers etc. Most 'cheesy' lists you see are generally spamming a single TAC unit (6th ed Wave Serpent full of Fire Dragons, White Scars Bikers with Grav Guns, Flying Hive Tyrants with Devourers etc.).
This is why the Daemon Factory build isn't very good in it's extreme form - sure, it has a gimmick where it can summon a 4000pts in a game or something, but since it has no real teeth there's not an awful lot you can do with it apart from just make more ineffective models. Lots of common 'spam' lists are actually really terrible for this reason; they're not spamming a TAC unit and thus end up with a glaring weakness (Typhus and his Zombie Horde, Green Tide, Tervigon Spam, old Cronair etc.). This ties in to the idea of an army having to switch between offensive and defensive play; spamming anything other than units that can effectively deal with all or almost all units in the game means you can't play defensively because your list hasn't got the flexibility to react to your opponent.