The apps are rather feature limited when compared to the Kindles. Most of the more interesting features simply don't work and I don't expect Amazon to enable them.
Now that I think about it, it might be easier to list what you cannot do. The Kindle has enough neat features that often times I only notice when they are not available. Let me know if you think I missed one.
Formats Not Supported
You can use the apps to read _some_ of the content that you buy in the Kindle Store , but not all. Here's a list of what won't work as well as several exceptions.
- Kindle Active Content
- Audible audiobooks
- Kindle Print Replica - Amazon's PDF format only works on Kindle for PC/OSX. (On a related note, K4PC can also read a DRM-free PDF.)
- Kindle A/V - while you can read the text from an enhanced Kindle on any Kindle or app, the video and audio only work on iPad and iPod Touch.
This is actually not as simple as I thought it would be because Amazon have been rolling out features 1 at a time and only adding them to a single app. It's a little odd but I guess it works for them.
Features Not Supported
There are a bunch of really neat things that you can do with a Kindle but not with the apps. Amazon haven't implemented any of these features and I do not expect them to.
- Emailing personal docs
- Pushing personal ebooks to the app from the Manage Your Kindle page
- Kindle Cloud (archiving of personal ebooks and annotations)
Given that the above features all depend on software, not hardware, there's really nothing stopping Amazon from enabling them. But I don't expect them to do so because it doesn't make good business sense. Giving the Kindles better features than the apps is probably intended to encourage you to buy a Kindle.
BTW, while you cannot email docs to your app through Amazon, there's nothing stopping you from emailing an ebook to yourself or emailing personal docs to the @free.kindle.com email address on the account. Open the email on whichever device you happen to be using and you should be able to download and open the ebook in the Kindle app. This doesn't always work though.
Did I miss any?