For a long time, I've had a theory about 40k army building, adapted from a popular saying about the roles of women and compatriots- Boys before toys. It came about from my Guard army, where in the old codex, it was very easy to spend tons of points on Doctrines and cool hardware, without remembering that at their core, they were still just regular guys with lasguns, a weapon that is really only effective en masse. It does apply, however, to nearly every army, and I played a game a few days ago that illustrated it extremely well.
2500 points, Ultramarines on both sides of the table. This kind of matchup is always nasty, as the Space Marine codex is certainly versatile enough to build many different armies, but ultimately you've got two armies both using the 40K baseline stat line, so it becomes less about the dice, as any kind of even fight is going to be a coin toss anyway. Then it comes down to stacking the odds in your favor with either numbers, or hardware.
Cato Sicarius and Command Squad (Apothecary, Company Champion, Company Banner, Thunder Hammer, Power Sword/Storm Bolter)
5 Assault Terminators w/ Land Raider Crusader (Sarge w/ Thunder Hammer and Storm Shield, 2 Lightning Claws, 2 TH/SS, LRC has Sergeant Chronus, Multi-Melta, Storm Bolter)
Venerable Dreadnought (Assault Cannon, Extra Armor, Drop Pod)
Tactical Squads I-IV (I and II have Plasma Guns, III and IV have Flamers, all four have Missile Launchers and a Sergeant with Bolt Pistol and Chainsword)
Scout Squad I (Sergeant Telion, Missile Launcher, 3 Sniper Rifles)
Scout Squad II (Sergeant with Power Fist, 2 Shotguns, 2 BP/CCW, Heavy Bolter, 4 Boltguns)
Land Speeder Storm
Vindicator with Siege Shield
Predator Destructor (Autocannon turret, no other upgrades. Discount tank!)
Chapter Master (digital weapons, melta bombs, artificer armor, plasma pistol, power weapon)
2 Dreadnoughts (Extra rmor, Storm Bolter, Assault Cannon)
10 Terminators (2 Assault cannons)
Tactical squads I-III (10man) Flamer, plasma cannons in I and II, Lascannon in III, Teleport Homers on Sergeants, Razorback with Lascannons and Extra Armor for III)
Land Raider (Multi melta, extra armor)
Vindicator with Sergeant Chronus, Siege Shield
Grey knight grandmaster and retinue (Grandmaster, NFW, Psycannon, Holocaust, Retinue x5 with Holocaust. )
Obviously there are some similar units in our forces, but in general, the opposing force is better equipped compared to my fairly numerous army. The game was a two-flag objective mission, and the opposing force mostly tried to camp their objective, while the front halves of my Tactical Squads advanced across the table, with the back halves providing covering fire. The Dreadnought dropped in right in front of his lines, doing a bit of damage before a lucky Plasma Cannon shot blew him up, and the Vindicator got into a game-long duel with his Land Raider.
The key difference was in the use of our Terminators. While giving his Sergeants Teleport Homers was a good move considering the two expensive teleporting units, he didn't move his Tactical Squads aggressively enough to make the homers worth using. That led to making risky unguided Deep Strikes, which resulted in his Grey Knights being destroyed, and his Terminators spending the rest of the game walking towards the action. My Terminators, on the other hand, enjoyed the comfort of their Land Raider until they were needed, keeping them safe, warm, and well fed (Didn't you know, Land Raiders have a minibar!). They were able to get out towards the end of the game and clean up, while the Crusader meandered around the battlefield blowing the crap out of whatever got too close to it. My investment in my Terminators was much less, and better protected, leaving more points available for the blue guys with boltguns that ultimately will carry the day for any Space Marine force (though they're not always blue, I suppose). Certainly I got lucky with the deep strike mishaps, but ultimately, do you want nearly half of your army depending on whether or not you manage to roll below average on scatter rolls?