When I was in college, I was the president of a student organization. Having pulled off what people came to call, "the only successful student coup" in the college's history, I had a big job ahead of me. People didn't come to our meetings, they didn't even know we existed. The previous administration had put up signs all the time and yet no one came. What is it that people weren't interested? I didn't believe so.
My initial suspicion was that people just weren't seeing the signs. The signs were up, yes, but I really didn't think people saw them. Why? For one, the text was about 14-point, a bit too small to catch a passer-by's notice. Increasing the font size would help, but I saw another problem. Basically, the signs would stay up until they were replaced by signs for the next meeting...same sign, different date, same place. The problem became clear: the signs didn't create enough difference in the environment to be noticed. They were visual noise, always in the same place, easily tuned out as part of the norm.
What I ended up doing was this: I tore down all of the original signs, waited week, then put up new signs in different places (with larger fonts, of course). After the meeting occurred, I'd tear down the old signs and not put up any new ones until a week before the next meeting. Rinse and repeat. What happened? Meeting attendance went up. The signs became something new in the environment, they stuck out because they weren't there before, and it's differences that people notice, not similarities.
This lesson can be applied to almost anything: people always notice what's different before they notice what's the same. Every once in a while I'll wear a tie to work, just for fun. People don't know how to react...why is he wearing a tie? Is it because he's interviewing somewhere? Is he meeting with someone important? The actual difference is small, I'm usually wearing pretty much what I always wear, just with the addition of this small sliver of fabric around my neck. But people notice it and react to it.
Taking this knowledge into the business world, you can easily understand why people hate upgrading. Upgrades are usually related to the versions they replace, just with new features (sometimes billed as "easier") and maybe a new interface. Perhaps most of the controls are in the same place, yet people notice the differences first. My old one did this, I used to just do that. It's a constant challenge to create an upgrade experience where the differences are subtle enough to be seen as improvements rather than disruptions. Change is scary, people don't like it, which is why my parents still have analog cable with an old box that is nearly falling apart. Upgrade to digital? What I have right now works fine, and something new might not do what I want. It's a hard argument to win.
People notice differences before they notice similarities, it's just that simple. The differences may be perceived as good or bad depending on the current state of the environment. If the current state is undesirable, then differences are seen as a good thing; this is the tact taken by salespeople when trying to convince you to replace a product with theirs. If the current state is desirable, however, then the differences are seen as a bad thing; this is why people stay in lousy relationships. When you know you're introducing differences, you need to understand the effect you will cause as well. Whether that be on a college campus, in the business world, or in your personal relationships, managing how differences are perceived is an important skill to have.