Expensive, Less features when compared to an equally priced Dell or HP, Not an easy Operating System, Not all softwares run on a Mac - these are the reasons why you shouldn't buy a Mac..
That being said, now lets analyze all the above reasons..

Firstly, Expensive. Definitely a Mac is very expensive. That's majorly because it was Steve's dream project and he doesn't wanna give that for free.. 2) Its from Apple - the brand that stands solidly on quality, perfection and innovation.. 3) The people who don't buy a Mac actually don't get to know whats really in a Mac. Let me explain this in detail - People generally consider Mac to be just another Laptop less the Windows OS. But its more. It has a lot of applications that ordinarily a windows PC doesn't have. Examples being GarageBand, FrontRow, iPhoto, iMovie, Automator, iDVD and more.. These softwares individually cost more when you buy them separately for Windows - say Photoshop, Nero Burning software, a decent video editor.. FrontRow is an exceptional piece of an application - that gives you a theatrical iPod experience.. All these and more softwares comes with every new Mac.

I initially tried to assemble a custom made Dell Inspiron 14" laptop in the dell website (following some deal in deals2buy) and added a few very basic softwares and guess what the total came to - $789 [Remember this].. Statistics and the surveys from the marketing world say here , here and here that Apple tops the list of all manufacturers.. Also I read reviews that told that Dell's quality has been degrading over the years and the life of a Dell laptop is a meagre 1.5 years before it becomes garbage. On the other hand, a Mac is seen to be more reliable.. [How many times has your iPod crashed unexpectedly ?? Believe me - it has never happened for me.. Given that I had been using mine for the past 3 years] Thats the reliability of Apple !

Coming to the OS - a very tough operating system.. Who told ya ?? If you are someone who likes Mozilla over IE, then you are someone who'd love the Mac OS X over windows.. Its a very cool operating system - professional and sleaky.. I love the graphics - the way it minimizes, magnifies and the likes.. Mac is like a hidden treasure.. There are ways to do it and its fun to go about learning it in the intuitive way.. Its like working on VI.. [If you have ever worked on it, you'll know what i'm talking about] After 12 years with the windows, I'm liking OS X better..

Not all softwares run on Apple.. This, to a certain extent is true.. Especially Google has been reluctant to release the Mac versions of Google Talk and Chrome.. But Apple has nothing to lose - it has iChat and Safari (needless to say Mozilla) to replace it. More the time Google takes, more will be the fan following for Mozilla.. the power of open-source. But most companies release softwares for Mac.. but some doesn't - especially if someone is in the engineering field they may need loads of softwares. Here's the catch.. there is something called boot-camp and VMware Fusion - that allows you to run Windows on a Mac - so that you can install applications that work only on Windows.. And the prize is this - Windows runs faster and better on a Mac. There are loads of other links that reiterate the same fact..

Now all these being said, let me tell you one more reason why a Mac is way better than windows.
Who makes the software (windows) ??
Microsoft.
Who makes the hardware ??
Uhmm.. Dell, HP, Sony, Gateway, Toshiba etc..

Coming to a Mac,
Who makes the software (OS X)??
Apple !
Who makes the hardware ?
Apple !

Don't you think if the same guys do both the hardware and software, they would know the notebook very well and how it would behave ? This is the reason why a Mac doesn't crash or crashes less often.
Here's a little suggestion if you are still concerned about the price.. Go for a refurbished Mac. When it comes to Apple, a refurbished laptop is just as good as new. It comes with the same 1 year warranty as the new one, the finish looks exactly the same as a new one. Believe me you can't spot a difference between a new mac and a refurbished mac. Except that the price would be 200$ lesser than the new one.

Last but not the least, there is one major reason for the vast popularity and the general tendency of people to prefer Apple - VISTA - read this post by Ab to know more..

If you want to learn more about the Mac - visit the Apple website.. Their site market themselves way better than any other site. These are just my views on why I sought to a Mac rather than a Dell.