A whole bunch of stuff comes together to make a user experience. This isn't just true in software, but rather in any industry that makes stuff. Most products that appeal to broad demographic groups have one of two qualities. One possibility is that the product comes in a few different flavors designed to appeal to distinct market segments. For instance, most adults in the United States own cars, but cars come with different price points and feature sets, such that minivans are for families with young kids and convertible roadsters are for empty-nesters with mid-life crises and disposable income. I don't know whether these market notions are really true, but the automobile industry makes a bet that they are and marketing campaigns are developed accordingly.
The other possibility is that the function or service provided is so universally appealing that the notion of market is widely inclusive by definition. Who buys milk, or toothbrushes, or socks? Pretty much anyone who buys anything buys these things, and while you do find some market differentiation even for these products (conventional milk or organic? manual toothbrush or electric? ten pairs of white cotton socks for $2 or one pair of silk dress socks for $5?), by and large you see less variation in this stuff than you see across products with a wider variance in possible price point.
Start thinking about the products that consumers want to personalize and things get more interesting. When I was a kid I had the same blue LL Bean backpack that everyone else in my class had and I wouldn't have thought of having anything else. But I also stuck stickers (puffies and scratch-and-sniff) all over the thing to make it mine. I wanted it to be the same, but I also wanted it to be different.
Think it's just kids? Think again. As adults we monogram towels and bathrobes. We paint our houses different colors, we hang art on our walls, and we add distinctive ringtones to our cell phones. This is all true even when we're all buying the same towels, the same houses, and the same cell phones as everybody else. Personalization is the name of the game.
Within the technology industry, Apple kind of got the hang of this from pretty early on, with different hardware form factors for different target audiences. There's no mistaking the group that the iPod is intended to reach. Within the software space too, we see all kinds of customization of user experience: different UI languages, different desktop backgrounds, different sets of applications and ways of organizing those applications (I need a clean desktop, for example).
But of all of this stuff, I'm wondering what really makes your computer feel like it's yours. Is it the picture on your desktop? Is it the post-it notes framing your monitor? Is it the way your hardware is organized, with an optical mouse on the right and an ergonomic keyboard in front of you? For me, for instance, it's a lot about how my files are organized. Hardware is mostly irrelevant, but if I can't find stuff, then I get lost, and if I get lost, I quickly feel like I'm using someone else's machine. But I'm willing to bet that the really key stuff, the stuff without which a computer just doesn't feel like it's yours, varies a lot by person.