Certain types of "scripts", "applets", etc. I traded in my old computer a while back, so I don't remember exactly. I'm no computer expert, but I know that I had trouble with a lot of things that popped up on websites when I had an older computer, and the geeks at my ISP's help desk attributed the problems to the old computer and/or its old operating system. In addition, many older computers lack sufficient speed and/or memory to cope with the heavy graphics that are commonplace on websites now, and some sites are set up so you can't navigate them at all with the graphics turned off.
It's not hard to upgrade a browser to accomodate scripts. And paying a little for more ram is a helluva lot better than selling your soul to Big Brother XP so a former fry cook can authorize you using what you bought and paid for. (Just wait 'til Longhorn comes. There's rumors you'll have to have yesterday's fry cook authorize you playing a .wav file. That's "progress"??)
And as far as graphics.... are you referring to that (typically) garbage called Flash? That's not the fault of the OS either.
Some of the problems you attribute to older OS's, I say is because basic common sense is increasingly vanishing, and site building morons are breeding like flies, (and I'm not the only one who feels this way):
http://www.webpagesthatsuck.com/
Addenda:
One other reason for "script" troubles isn't the fault of the OS or browser or the computer, but simply because there are morons who build sites that don't take into account the intelligent souls who run firewalls!