My dad uses his iPad for work (esp. for Dropbox + Adobe Reader) because it boots up faster than a laptop which saves time at meetings.
For myself, I find that I would have no purpose for an iPad other than to watch movies, play games, read books and listen to music or audiobooks--so I don't see why I need to pay out the butt for iPad features I won't use and don't care about (well, I do
have a Dropbox account, but would I use it on an iPad? No. Why?)
Also,
from what I understand, the iTunes store is bigger, but not by too much, and there is greater reach of Amazon software throughout non-Amazon devices (eg. Kindle App on iPad). Also, Amazon's software allows for instant streaming of content. Media bought from iTunes does not, except for on AppleTV. Yes, I can stream Audible audiobooks to an iPad, but that's because Audible is owned by Amazon.
Suffice it to say, if you want to use Apple's software, you're required to use Apple's hardware and iTunes. With Amazon, you're not. Sure, if you have an iPhone, an iPad, an AppleTV and iTunes you're set, but I don't.
EDIT: Oh. Also, my wife and I play an MMO called RuneScape, which is entirely java-based and can be played in a browser. Can I play it on an iPad though? No. And, come to think of it, can I watch Stephen Colbert streaming from Comedy Central's website on a iPad. Nope. I'm not sure I could do these on a Kindle Fire either, but this means all I'd do with an iPad is browse Facebook, check email, listen to audiobooks or music, read books, watch Netflix and play Angry Birds or Fruit Ninja. How is that better on an iPad than on a Kindle Fire?