With plethora of OS related changes, Windows Phone 8.1 brings with itself many much-needed “apps purchase and reinstall” improvements as well, it seems. So, now it will be possible to see the owned apps in “My Apps” list and you will be presented with a “install” than a “buy” button for already purchased apps, when you reinstall. The apps will also have info about when you purchased that app, as is clear from below screenshot.
There is one more interesting improvement. Now in-app-purchase can be possible in landscape orientation as well.
Other improvements like apps on SD Card and automatic apps updates are known, but here are screenshots from official slides.