A long time ago I wrote about the fact that writing is like composing; change one word and you can change everything. Even if most people can't articulate why the words feel like they mean something different, even if they can't actually see that there is a difference, it's enough that I know and feel that there's a difference, and I'm going to try my damnedest to articulate that change so you understand it.
I forget sometimes that the same is true visually. When it comes to the web (as everything else) there are rules of design and alignments and cues that actually matter; it sounds goofy to say, but those details are the difference between creating something that users know is trustworthy, and something that just doesn't feel quite right. And that's not a feeling you ever want users to have.
When those details are working right, you actually don't notice them; you're simply using and enjoying the site as you want. You're focusing on what you want to do.
When they're not right, when they haven't been considered, or have been forgotten in the clench of compressed timelines and budgets, you become conscious of the process of using the site, and vaguely critical of it. You've been taken out of yourself and what you want to do.
And you unravel everything that you've been trying to do.