1. Updating will be a huge pain
You do realize that you can't just pop in the disc and install the OS, right? Coming from XP, you're going to need to backup all of your data, format your hard drive, install a clean version of Windows 7, and then start from scratch, reinstalling all of your old programs—and that's assuming Old Faithful even meets the system requirements. Sounds delightful!
2. Software investment
How many programs do you have installed? You're going to have to reinstall all of them. Do you have all of your install discs handy? And I hope you haven't lost any CD Keys! Do you still have all of your downloaded installer executables? Feel like finding them or redownloading them? Because that's what you're going to have to do. And as far as new programs go, you do realize that almost all new software is still compatible with XP, right?
3. Most of what you use your computer for doesn't need an upgrade
What do you do on your computer? Surf the internet, maybe use some office programs? I'm going to go out on a limb here and assume that Windows XP has been handling such duties just fine for you. So why upgrade? For shiny new aesthetics? For a fancier taskbar? For juggling 22 devices? You don't need that.
4. It's expensive
Do you have $120+ to burn? Because that's how much upgrading will cost you unless you use the $30 college-kid discount. Why not put that money in savings or use it to pay off a credit-card bill, like a grown up?
5. You can wait for SP1
Every OS has bugs when it's first released, and even if 7 isn't the shitshow that Vista was, it'll surely need some patching up once the masses get their hands on it. You won't get any bonus points for being an early-adopter. Why not play it safe and, if you do decide to upgrade, hold out for Service Pack 1?
6. Microsoft will keep supporting XP for a while
Tons of people (including you) still use XP, and will continue to do so for the foreseeable future. Because of that, Microsoft has no choice but to continue supporting it, releasing security patches and the like. You don't need to upgrade in order to get such benefits.
7. You'll buy a new computer eventually
You won't have your current computer forever, especially if you bought it long enough ago to come pre-loaded with XP. Since installing a new OS is one of the most risky and frustrating things you can do with your computer, you might as well just hold out until you buy a new one. It'll have Windows 7 pre-installed on a clean drive, allowing you to start from scratch.
If you're running Vista, however, you should by all means upgrade. What are you, crazy?