A lot of computers (especially older ones) generally do not provide enough power to their USB ports to charge the iPad at a sufficient rate - it may still be charging slowly, and should be a bit quicker if the iPad's screen is off (if you leave it for a while and then disconnect it from your computer you may find that it's battery percentage has gone up). The wall charger is usually the most efficient way to charge the iPad.
In terms of the wall charger, if you've got an iPhone or iPod Touch then you're not using their charger are you, as they are less powerful ? If not then try a reset and see if it charges after the iPad has restarted : press and hold both the sleep and home buttons for about 10 to 15 seconds (ignore the red slider), after which the Apple logo should appear - you won't lose any content, it's the iPad equivalent of a reboot.
Otherwise have you got a different cable that you could try, or do you know anybody else with an iPad so that you can try their cable and charger and they can try yours ? If not then you might need to go to a store.
Here is another question: is the data sync cable that comes with the ipad different in any way from the ones I have for my itouch and various ipods that have died? I see an off brand one sold on Amazon that is 6' long which I would love for my ipad, but a few reviewers are saying it isn't strong enough to charge an Ipad. I thought the part that plugs into the wall was the issue there, not the detachable cable. Educate me please!
The Apple cable is the same - they used to be different, some older iPods had firewire cables instead of USB, and the end that plugs into the device has changed design but mine still work on all devices, but they are interchangeable. I currently use a cable that came with one of my old iPods with my iPad, iPhone 3G and iPod Touch for connecting to my computer and connecting to the wall charger (which is different for the iPad).
That's what I thought. The critics are wrong, they just want to drive down product rating for whatever reason! Wonder why Apple never made a longer cable? I can see you would have to be careful with it, it would be easy to drag your device to the floor, but having one at 6' long would be useful. Thanks.