Ok, here are the biggies;
1. Linux is free
2. Updates are free
3. It's robust, it won't blue screen on you (ever! - if you have working hardware)
4. It won't get infected with a million viruses every time you use it
5. Generally speaking it'll feel quicker than windows, not least as the multi-tasking is better
6. If you get a faster processor, more processors or more cores, you'll never have to pay for a license uplift
7. If you chuck it on an additional machine, you won't be breaking any T's and C's
8. You'll never have to call to register your software after you've installed it or be told you've run out of registration attempts
9. It won't have to donate 75% of it's system resources to virus checking
10. You'll never have to run 'defrag' and you'll never slow down because it's not been run
11. You won't have to wait 3 minutes while your machine boots and check for Virus updates
12. You'll be able to visit sourceforge.ne
t and download packages for free (currently 230,000 available, most are for Linux)
13. You'll have a more technically competent system
From a basic standpoint, you'll have a mouse, Windows, web browser, mail package, (free) Office package with wordprocessor, spreadsheet, database, presentation etc .. chat client, IRC client, Graphics tools (photoshop-alike) , Photo manager, your camera should plug in, as should your webcam, etc etc .. it'll browser Facebook, play Youtube videos, and give you access to many things you may not currently be aware of ..

The possibilities are endless .. if you imagine all the software that's out there for Windows, then multiply the number of packages by 100, then make it all free .. then figure the software is written mostly by people who love coding and are really good at it and who are doing it for enjoyment, rather than to make a fast buck ... (!)