I was chatting with a buddy, and it made me want to blog about the impact of terrain rules in 40k on game balance and list design, as well as on the tournament scene.
Line of Sight blockage of a substantial degree has been a meaningful component of 40k for decades. Focusing just on the more recently memorable 4th Edition, however, you see a situation where MOST terrain and vehicles provided significant LOS blockage.
Coupled with the ability to consolidate into new combats, 4th edition was much stronger for assault armies than 5th on a baseline level. Removing the ability to consolidate into new combats shifted this back the other way, but the change to line of sight rules in conjunction shifted it too far.
The key here is most terrain was ridden with windows and line of sight gaps prior to 5th edition. Heck, plenty of players in their homes simply used felt circles and such to represent stands of trees and other things that effectively were line of sight blocking regardless of height or tree density. With the release of 5th edition, thousands of players worldwide suddenly had a dramatically different effect from trees, buildings, and anything else that wasn't a solid block of material.
When you think about the balance of combat and shooting, a significant component of it is the simple fact that in a line of sight rich environment, shooting units are active EVERY turn, whereas assault units need to often times cross considerable distances to start using their offensive capability. This is a large reason why assault units that are cheaper, and/or faster, and/or fearless, and/or more durable are a better investment, and a requirement ... if you can't get the unit there AND still do damage, you're never going to compensate. Line of sight blockage corrects this to a degree, as does/did the improvement of the "average" cover save to a 4+.
One of my biggest concerns for 6th edition is reducing the "base" cover save back to a 5+. Doing so would directly benefit further MSU armies and "bullet spam," because torrenting non-marines out of cover becomes that much easier to do.
Fortunately, as 5th Edition matured and newer tournament styles and approaches came out, line of sight blockage has improved steadily in terms of what you'll encounter when you travel to a competition. This, plus very especially the nightfighting and movement control instilled by Necron are causing a "break-up" of the old "shoot them until they die and don't take deathstars / other units they can just draw a bead on and torrent over and over" mentality. If boards have significant LOS blockage and you are using a pure MSU army with tons of rhino or chimera hulls, you have a harder and harder time moving all of your vehicles a few inches here and there into position to draw shots on KEY units (i.e. draigo deathstars, straken type aura generators, etc.). Additionally, NON marine armies are able to use a 4+ cover save to make up some of the gap in power level that is semi-inherent to the nature of half the armies in the game being some kind of marine/3+ variant (CSM, SM, DA, BT, SW, GK, BA, SOB, kinda-Necron).
I'm rambling a bit as standard, but - the long and short is this:
There's a fine balance to be had in keeping a game from becoming stale, where "normalized" builds are the only ones that work. As 5th edition matures, line of sight blocking becomes more common in a tournament setting, and new codices like Necrons break up the notion of what really makes a "best" build (i.e. razorwolves / vet+hydra spam doesn't work well if there's line of sight blockage at all in an environment that includes tremorcrons and scarabfarms) ... the result is a game that becomes increasingly interesting, and far less predictable at the tournament level. I think most people will more and more come to like this.
If you ARE a fan of this, you want to hope things like the rumored 5+ cover save reduction in 6th edition do not come to pass ... anything that further decreases the ability to protect key units or keep more fragile units covered en route to where you want to go on the board will de facto INCREASE the common nature of MSU spam shooty armies. While these for a time (and now fading some) were the "best" statistically on a non-LOS blocked game board of 5th edition 40k, I think most people who become more and more veteran with the game can agree that they don't necessarily have a positive impact on variability and fun. This isn't chess, after all, and the more a game can mature and settle into a situation of high variability, the more interesting and unexpected the tournament scene becomes. This further creates a situation where unique and balanced armies are ALWAYS better to take and prepare with than copycatting an internet list, rolling lots of plasmaback and missile shots, and blaming losses on your bad rolls.
Ramble a bit, but hopefully some interesting subject matter.