Until you get into Linux you won't know how much of a pain closed source can be... In Linux there aren't really any license agreements, and it's free. Software isn't something you buy, it's a module you add to your computer for additional functionality. You buy the computer, and that's it. Just the way it should be.
Here's how the software situation is in my experience: you have it or you don't(and you have most things). If you have it, it's free and JUST as good as something you pay $150+ for(i.e. open office) I can't comment on GIMP vs. Photoshop, but I know a lot of GIMP lovers. Can anyone name one function that they use all the time on Microsoft Office that Open Office or AbiWord doesn't have? Most people I know don't even use serious functions, they just do basic spreadsheets and basic typed documents with simple formatting. VLC and MPlayer are top notch media players which originated on Linux and became cross platform(and all I use on my Mac under Leopard).
To me, license agreements are a very big thing on the OS, and likewise it's very big that Linux doesn't have one. If I have a Linux disc I can use it an infinite number of times on an infinite number of computers. If I have a Windows disc, it only "legally" works on one. If I just want to run a Windows virtual machine for one Windows app, I must buy a full Windows License. This is just flat out wrong. And, it's the same for OS X(though it's harder to notice, since it only officially runs on actual Macs anyway).
Drivers are also far better on Linux, it doesn't always support the thing that came out yesterday, but if it has ever supported it it will continue to do so for eternity, far less than I can say about Windows or Mac OS X. What's more, the support will increase in quality over time, with frequent updates. Look what happens when you try to install Windows XP on a Vista laptop, you have to find all the drivers yourself. On Linux, on the same laptop, everything else will most likely work out of the box, other than the Wireless, which is usually just a Vista driver download and installing it in Ndiswrapper. If it's an Intel wireless chipset, or you're using a distro that ships lots of proprietary drivers, it might even work out of the box.
On Linux, you can choose the interface. If you like the Mac's interface that's great, I don't think it's bad, but I can always think of something that could be better whenever I look at an interface. On Linux I'm free to make it better, I can install a different interface, and the more robust ones allow you to customize and rearrange bars, buttons, menus and just about everything in between. There are many Dock-like applications available for Linux if you like those too. I can even choose to use a pure command-line interface, the only one of the big three OSes that lets me do this.
It's the fastest OS, the Mac has somewhat of a lag time starting apps for the first time since boot, Windows is laggy in general if you're running background processes(anti-virus, spyware, helper applets, etc.), Linux is utterly smooth even on modest hardware, with smooth anti-aliased fonts and slick theming. Other than Firefox with lots of extensions, I've never seen something take a delay to start on Linux, you just click and there it is.
And of course, the server. Once again, it's the fastest OS there is, licensing is even more complex for server OSes, and Linux once again involves none of that mumbo-jumbo. There isn't even a real distinction between a server Linux and a desktop Linux, other than what packages a distro ships with.
Possibly the most important thing to me, it's SIMPLE. Not Mac simple, which isn't really simple, that's "easy to understand for the non-technical user." "Simple" is generally the opposite of that, simple means there's not a whole lot going on in total, and it's easy to know all the directories and files in the system off the top of your head. It's simple at the level of implementation, rather than the level at which it is used by an end-user. It's so easy to get things done with Linux once you're acquainted with it, because it's logical, all the config files are here, all the logs are here, all the user stuff is here, all the programs are here, etc. etc. You know what something is just by looking at it's name and where it is in the directory structure. Nothing is hidden from you in Microsoft or Apple only areas.
I like my Macbook, but I'm not sure if I'll get another Mac for when I upgrade my Desktop. Linux works perfectly for me and my family's needs, without anywhere near as much fuss as Windows, or the relative expense of a Mac. Not paying for software and not pirating is a Good Thing.
In summary:
1. Free
2. Works just as well for 99% of what people use computers for
3. Freely distributable, no licensing worries
4. Everything is laid bare, you can truly understand it
5. Simple implementation
6. Fast fast fast
7. Generally secure
8. Easy to add applications and remove them(and once again, free)
9. Less driver worries overall(it just seems like there's more, because it doesn't ship on computers)
10. Doesn't break unless you break it
11. A server you don't have to pay crazy licensing for(No CALs)
