This is a summary of the most important changes and updates to Windows 8.1 for developers, from my point of view.

Live Tiles

Two new sizes: 70x70px and 310x310px. I especially like the new small size which makes it possible to create a start screen that looks more like Windows Phone 8.

New and improved controls

Yeah, Date and Time picker controls and the new Hub control. The Hub Control makes it easier for apps to use a hub-like navigation pattern.

New windows states

A more flexible model is introduced in Windows 8.1. Now you can have more than two apps on the screen at the same time and one app can use more than one monitor. The minimum size of an app is now 500px. Snap mode is optional, 320px.

Other things

New features for camera capture. Support for WiFi-Direct that enables a direct high-band connection between devices. A new HTTP API for apps using HTTP and REST. Prefetching HTTP content before it is needed with HTTP prefetching

This is, as I have already stated, only a small summary of changes and updates. See here for a more complete list: Windows 8.1 Preview Product Guide for Developers.