It didn't occur to me to address setup and the like. I take a page from Hannibal's book most of the time, and go for a weak center and strong flanks. At first glance, it looks like a glaring deployment error, since you don't want to split Space Marines up. However, the weak center (which isn't that weak, really, but it's just not as heavily populated) can react to either left or right pushes, as I tend not to take static units. Enemy units have a choice to either react to the left or right flank rushes, which are generally pretty balanced in terms of overall threat. Moving to react to one side leaves the opposite flank open for an uncontested charge in, while trying to react to both flanks at once usually leaves the enemy spread too thin or compacted to where my superior assault units can easily consolidate into new combats, eliminating the ranged threat.
That being said, that setup doesn't work against Tyranids, mobile assault armies, or Necrons. In the case of Tyranids, you're on the defensive by their very nature, and staggered firing lines are an excellent approach. With mobile assault armies, it's a question of outmaneuvering and charging them, rather than the other way around... it's just something that has to be played by ear. Necrons are tough enough to hold a strong center with a phalanx setup, so you kind of have to play on their terms. You'll take some casualties marching into them, but it's a hell of a lot safer than using Rhinos to get to them-- Gauss weapons will stop transports dead in their tracks. Fighting Necrons is, unfortunately, a war of attrition, and they're better at it than anyone else.