For the sake of discussion, we'll use me as our example. I build competitive lists. Put another way, the lists that I build take whatever form is necessary to create an effective Grand Tournament army against all-comers. So I lean away from all the sub-optimal choices in a codex in lieu of ones that give me the necessary pieces to create a balanced list. There are many ways to do this and I won't get into them here. However, because of my stance on competitive lists, I very rarely use thematic units or armies. I also tend to stay away from close combat as the main focus for our Black Templars lists, instead utilizing it as a strategic tool to win games. So, as you can see, I have some bias.
How does this show up in my lists? Well, the most obvious one is the fact that I've built and used a BT gunline list to great success. So successful in fact, that I am considered the architect of that style of list. Other players have emulated the list over the years and produced similar results. The list focused almost entirely on the shooting phase to win games. Pin the opponent in their deployment zone, then use a combination of mobility and overwhelming firepower to win games. Nova Open and other GT's presented me with an unique problem however; there were large amounts of line of sight blocking terrain. It prevented me from playing in the typical manner and thus, the gunline list suffered.
But that isn't the only tendency that I follow. My lists will, more times than not, feature massive MSU in my unit choices. I strongly believe that an overwhelming presence gives you many more tactical opportunities on the battlefield and presents a challenge to your opponent. Even the most powerful list will have difficulties eliminating or neutralizing every last unit you have when there are a great many of them on the board. Adversely, if you have few units, your enemy only has to allocate a small portion of their resources to eliminating each. I'm not really going to discuss the KP discussion point here, but I believe it to irrelevant in most tournament environments (i.e. Nova Open, ITC).
So this brings us back to square one: is this hurting my ability to field better units in my army? I want to say that everyone, including myself, will say that they have an open mind and would choose the best tool for the job. But I think that assumption would be overly optimistic. With 8th Edition released and a new codex on the way, we get to hit the reset button on a lot of our long-held list building tendencies. And I guess I am happy about that.
What do you do with your lists? Do you have tendencies that you follow? Are they rooted in fluff or competitive reasons? If you're losing and don't like it, is changing some of your preferences an option? Let's hear some of the self-analysis here.