There are tons of reasons why Linux is preferred by some people over Windows, but first I want to point out a few inaccuracies in your arguments:
1. A lot of linux users build their own computers, so they don't "come with Windows".
2. There's no such thing as "the Linux user interface." There are about a dozen mainstream interfaces that are linux compatible. If you don't like the one that came with Ubuntu (incidentally, I don't blame you: the Unity interface is horrible) then just install a new one. Unlike Windows, you are not stuck with a single interface. For example, on my wife's user account she kept the default Unity interface, but on my user account I'm running the LXDE interface, which is faster than Unity and more closely resembles the Windows interface.
3. Once again, "Slower" is an Ubuntu problem, not a Linux problem. And again, if you don't like how fast your current installation is running you are free to tweak it and tune it to make it faster, which is also unlike Windows. In most environments Linux is actually much MUCH faster than Windows.
Now on to the main reasons people like Linux better:
1. It's open source. This means you are free to alter the program any way you want and even distribute your own version of Linux and nobody will sue you. I know that doesn't matter for a casual user, but in the programmer's world that's a big deal.
2. It's way more secure. And I don't mean that fake "nobody writes viruses for it because of low market share" type of secure that Apple fanboys get away with. I mean it is truly more secure from the kernel up. Basically as long as you're not stupid enough to give away your root password its safe.
3. It's way lighter. Meaning, it runs faster in most situations (like I said before, your case is unique in that your dad chose a crappy distribution).
4. Customization. This feeds into #1 above but on Linux you can tweak EVERYTHING. I know some users like having a system that "just works" and don't care about customization, but there are those of us who like tuning our computers to the absolute best performance.