Microsoft released new polished version of windows 8.1 preview this is major release after launch of windows 8 . it has some major changes , one of them is START button , Yes , now windows 8,1 will have start button and also it will have option to directly boot into desktop session ( without opening start screen ) 

Search is optimised for windows 81. to search almost everything in your pc and online

Search once. Go anywhere.
A single search now brings you results from your PC, your apps, and the web. See results in a clean, graphic view that lets you take immediate action and launch apps. Search, click, do. Find a song and start playing it, or find a video and watch it right away. Powered by Bing.

and customisable start screen background . it will also have Internet Explorer 11 ,  Internet Explorer 11  Preview is built for touch, with faster load times, a full-screen experience that includes side-by-side browsing of your sites, and real-time info delivered on your Start screen through live tiles for your favorite sites.
skydrive is now improved , app store improved for better user experience . it comes with so much more than that,so its up to you how you use it and discover its features .

Downloadable ISO file are  not available ( at the time of writing ) but as they announced will be available soon