It's odd that your subject line should read: "How would you sell me Linux", when generally speaking, it's not for sale.

Unless of course you need enterprise level, and believe me, that *does* cost a tidy sum.
Let's see what more confusion I can add to the mix.
I have always been curious about different operating systems, I have used a mac but i have always been on my Windows OS.
1)So what can Linux do that windows can't?..
Brutal honesty. It really depends what you intend to do with your machine. To categorically state that one machine can do more than the other, or to make the assumption that one use is more important than another, regardless of the individual requirements is ridiculous.
That does not answer your question, however more often than not zealots on either side will answer with a statement of what "their" OS can do and not really give thought to the question, or the intention behind the question at hand.
*nix -- and I will from this point on use that designation to describe all the vast multitudes of linux distros and the *BSD variants -- handles quite a few functions; some are the same you might use your windows machine for, others are not. Break yourself down into one of three categories; Gamer, worker, a worker who somehow finds time to play games. Further, you can break up the latter into the type of work you do. I will not delve into specifics, as I think you get the idea.
Yes, there are *some* games available for *nix -- generally these are older but well respected editions; rarely cutting edge. More often than not, these new games are not even available for Mac-heads; another group of smaller but more ferrel animal.
Ergo, if you want to play games... you're stuck with Windows.
Workers. Everyone who makes a dollar off their computer can be considered a 'worker' for the purpose of this argument. Therefore, especially given the forum, I think everyone here applies; though some of you may fall into the third category... if so, tell me how it's done.
Historically, and I will digress for a moment, graphic artistry at least form a digital standpoint was primarily the venue of the Macintosh. It wasn't so much the processing power, as it was a perception by software manufacturers that the Mac from an OS standpoint, was more suited for desktop publishing. Hmmph. Where there's smoke, there's fire. Programmers overwhelmingly bought into this concept and desktop publishing was expanding to the additional realms of digital pixel based media. Having what was simply the finest software available only for a particular architecture is a compelling argument for any graphics company looking to purchase new equipment.
Once a thought becomes implanted, it's difficult to remove...so despite the fact that nearly all the professional grade software available for the Mac is now available for Windows, seems to matter little; though a grass roots campaign is growing. The failings of one, become the groundswell of another. Where Apple made strides in production of software, they lagged in true network architecture...MS did not. (They yanked the FreeBSD TCP stack). Everything is network; network is everything. So we may yet see a reversal.
Ok, I really will answer...I promise. *nix can either do more than Windows or less, depending on your needs. If you need a box to run Adobe Photoshop -- Windows, (or Mac) is it. [Comment: Odd to note that now Mac's run a version of BSD called Darwin, running an old mach kernel -- one wonders if *nix versions of famous graphic programs are not far off?]
You simply have no choice than the status quo for most application-ware. Therefore, if your intentions are to make stunning graphics and design web sites using Flash... You have no choice, and there's no reason to switch to a Linux desktop.
However, if you intend to make services available; a web server, (or cluster thereof), portals, application serving nodes, ftp servers -- anything beyond layer 1... *nix is really the best way to go.
Why? I thought you would never ask.
Let's forget a moment just how vulnerable MS products are, if at all possible. That's right, put away your Symantec AV program to kill the latest worm infecting your windows system and pretend.
Good.
Let's talk about stability. MS has a theory, that theory is binary package control. When you run an update, you are basically patching a binary with a binary -- in essence wrapping a shell around the core. That shell redirects or otherwise handles the 'miscommunication', as they put it once, and is supposed to provide a reasonable level of repair. Hmm. What happens when you install 300 hot fixes? SHell, upon shell upon shell, and the inevitable BSOD. The system simply gets confused. I won't even go into the mess that is the 'registry'.
As many of you have found out, once in awhile you have to reinstall your OS to "clean things up" or, if it crashes once a week we think, "well that's just windows". We are like pavlovian experiments. We are conditioned to believe that is normal.
As a counterpoint, I have had under my span of control, *nix boxes that ran non-stop for over 3 years with NO downtime.
Now, as far as the distros of Linux go...some are following the dark side, (breathes heavily) and performing the same binary package install for patches. Hiss! Boo! If one, and one usually does, have the opportunity to rebuild the system and the kernel, it is ALWAYS the best option, IMHO. You cannot beat that stability. The argument from beginner sysads is of course, "what if I have 100 machines? Binary installs are easier to manage." My answer -- learn how to script.
So... the point being, if you intend to work and that work involves services, *nix can provide you with the most important thing you will ever need -- stability. Applications will ALWAYS take a back seat to stability. When we're stable of course, we mean secure as well. Not to say there have not been "problems" with any of the *nix distros mind you... but not to the order of magnitude MS seems to wantonly acknowledge, much less attempt to cover up.
My favorite tagline when I worked for the government in security was,
"My other computer is YOUR windows machine." Kills me.
If you wish to know more details -- yes by god, there's more

-- ping me personally.
2) I have heard about patching kernels...(wtf

)
Oi. Patching. We discussed that. Linux of course makes a distinction between the kernel and the OS -- BSD integrates the two. Either way, "patching" or updating are inevitable. It's not something you should do, it's something you must do if you intend to run services. Rebuilding a kernel is not such a daunting task as many in the field would make it sound -- like some inane 'right of passage'. These days, it's cake, with that wonderful buttercream frosting all over it.
Basically, and this is from a FreeBSD perspective, you add the things you want, remove the things you don't. When a LINT file comes out describing changes or updates, you add what needs to be added, and you type two lines. That's it. Well, 4 if you could the building of the world, but you stated kernel.
3)why do people prefer to use it?
Well, for the above stated reasons. Stability, which equates directly to security and, if I failed to mention, scalability. This is of course, as a serving platform. Why do people use it as a desktop replacement?
If it meets the needs, then why not have a stable environment? There are also a lot more cool tools out there for *nix, that you simply will not find for Windows. Some have crossed over, but many have not. Some are downloadable as raw 'C' code; every *nix box comes replete with a c-compiler -- gotta pay for one with Windows. I use it nearly exclusively, and if it were not for work requiring me to run Lotus Notes, and our damn VPN tunnel program provided by ATT as Windows only... I would be off Windows entirely. Now if someone could teach me how to fit a few little games in the mix, then I might consider retaining one Windows machines. The others, however, are all FreeBSD unix.
I am very curious about your answers. I have been using windows sooooo long, I would love to install linux and play around with it. So go ahead, sell it to me
So, I am not sure I sold you; I am sure I bored you to death. Sorry... sorry to everyone who actually took the time to read this diatribe. I get carried away sometimes.
Cheers.[/code]