Tom Warren from The Verge has a summary of the announcements. There is a lot of improvements under the hood especially to make developers’ job easier. This is Microsoft’s attempt to attract them and increase the number of good apps available, something iOS is ahead on.

From a user standpoint, the update to the Start Screen looks pretty neat:

The main Windows Phone 8 user interface change that Microsoft is showing right now is its improved Start Screen. "We're going to have three sizes of live tiles, small, medium, and large," says Sullivan, pointing out that the existing ones in Windows Phone 7 are medium and large versions. "The small tiles will enable us to fit more on the start screen, and to have things that don't require a lot of real estate to convey information." Inline with this change, Microsoft is also expanding the screen real-estate that you’re going to be able to place live tiles on. The trough along the right hand side of the Start Screen will now be filled with tiles. "It's going to fill up the whole screen," explains Sullivan. Users will also get the ability to customize the size of every tile, so each one can now be small, medium, or large.

People will be able to decide what app is more important on the start screen and reduce the size of less important apps. Very useful.

Watch the video if you want to have a better idea.