If you do a quick search you will see a number of similar posts.
Still don't see a real answer, though.
Perhaps the charger is a little undersized for the higher capacity battery, who knows. As long as it charges.
And tehre have been some users with issues charging from a USB port that had worked for prior models but not the new iPad. Could be related to the larger capacity battery.
I thought the same thing when I got my new iPad.
My son has a galaxy tab, and I have a xoom. We been using these I think like 2 years now, and they take about an hour maybe even less to charge full from 0%.
The first time I tried to charge my new iPad I thought something was wrong, since the battery does not last longer or even as long as my android tablets, I thought for sure it taking 7 hours to charge there was something wrong.
But after investigating I found this was normal.
To me the retina display is not worth the insane charging times, and I can't even turn the brightness on full or I get maybe an hour out of the iPad. Where as I can turn brightness full on my android tablets and get an easy 8 hours.
I don't think I'll be buying another retina display Apple device.
The quickest way (and really the only way) to charge your iPad is with the included 10W USB Power Adapter. iPad will also charge, although more slowly, when attached to a computer with a high-power USB port (many recent Mac computers) or with an iPhone Power Adapter (5W). When attached to a computer via a standard USB port (most PCs or older Mac computers) iPad will charge very slowly (but iPad indicates not charging). Make sure your computer is on while charging iPad via USB. If iPad is connected to a computer that’s turned off or is in sleep or standby mode, the iPad battery will continue to drain.
Apple recommends that once a month you let the iPad fully discharge & then recharge to 100%.
How to Calibrate Your Mac, iPhone, or iPad Battery
At this link http://www.tomshardware.com/reviews/galaxy-tab-android-tablet,3014-11.html , tests show that the iPad 2 battery (25 watt-hours) will charge to 90% in 3 hours 1 minute. It will charge to 100% in 4 hours 2 minutes. The new iPad has a larger capacity battery (42 watt-hours), so using the 10W charger will obviously take longer. If you are using your iPad while charging, it will take even longer. It's best to turn your new iPad OFF and charge over night. Also look at The iPad's charging challenge explained http://www.macworld.com/article/1150356/ipadcharging.html
Also, if you have a 3rd generation iPad, look at
Apple: iPad Battery Nothing to Get Charged Up About
Apple Explains New iPad's Continued Charging Beyond 100% Battery Level
New iPad Takes Much Longer to Charge Than iPad 2
Apple Batteries - iPad http://www.apple.com/batteries/ipad.html
Extend iPad Battery Life (Look at pjl123 comment)
New iPad Slow to Recharge, Barely Charges During Use
Tips About Charging for New iPad 3
Prolong battery lifespan for iPad / iPad 2 / iPad 3: charging tips
iPhone, iPod, Using the iPad Charger
Install and use Battery Doctor HD
In rare instances when using the Camera Connection Kit, you may notice that iPad does not charge after using the Camera Connection Kit. Disconnecting and reconnecting the iPad from the charger will resolve this issue.