Playing a tough looking list, a list that brings a little discomfort into your opponent, can often win you the game long before you ever deploy. At this point, you've got your opponent playing your game at the table, as he adjusts everything to face you. The worst I've played, my opponent took a 45 minute deployment, as he adjusted and re-adjusted his models. About half way through I decided its time to take a bathroom break, and best to just let him do his thing. Each army has its own fear factor, and honing your army for this is nothing something you should set out to do. Its more of a side thought, a final touch, that takes your army over the top. The end result is an army that your opponent has no idea, or no real idea on how to beat it. Sometimes it's the shock factor, or the massive number of tanks. What it has to say before the game even starts is, this is going to hurt.
There are two basic ways to demolorize your opponent, one is pre-game with your list or set up, and the second is hitting someone hard enough to break his train of thought. The end result is the same, getting your opponent to play your game, and not his. Having them react to you, instead of you to them.
Beyond your list, and deployment, there are other ways to demorolize your opponent during the game. One of my favorites, is being able to take down a transport, and then wiping out the entire unit that was inside with the next shot. Example... Hydra squadron blows up the rhino, Basilisk or artillery battery kills everything that came out. Another example might be, Assault terminators assaulting multiple units or tanks and destroying or breaking it all. Having a hammer in your list, really can bring home the victory in one fell swoop.
Coming up with your hammer is part of what makes list building fun. Seeing it in action is part of what makes playing fun.