Windows Phone 8.1 may require you to upgrade your microSD card, three column support limited
With these updates it is likely we will see the ability to install and run apps off a microSD card if the developer so chooses. To make that worthwhile however, Microsoft had to set some specific requirements for the use of memory cards in such fashion, most notably for speed.
The minimum level of support for Windows Phone 8.1 will be Class 6 SDHC, but the recommended storage is Class 10 SDHC. If you have an older card you have managed to keep working for the past few years, Class 4 and below will not be supported.
Naturally, Windows Phone 8.1 will have the ability to detect and determine if your card will work or not. If the card is fast enough, users will be offered the option to move apps to the card. This will also likely help with video performance when those files are stored on the card, thus minimizing jittery performance.
As for three column tile support which will be widely available for most existing Windows Phone 8 devices, there will be some limitations there as well. If you own a Windows Phone with a screen size under 4.5-inches, you will not be able to use the “more tiles option” when you upgrade to Windows Phone 8.1.
That will affect devices like the Nokia Lumia 520, Lumia 525, Lumia 620, Lumia 720, Lumia 820 (along with its cousins the Lumia 810 and Lumia 822), Samsung ATIV Odyssey, Huawei Ascend W1, Huawei Ascend W2, HTC 8X, HTC 8XT and HTC 8S
The issue is clearly not just a matter of screen size, but also screen resolution. All of the aforementioned devices, with exception of the HTC 8X, have 480x800 pixel displays. The “more tiles” would just present extra-small, illegible, items on the display.