First a caveat: Windows 10 is not even a product yet and won't be until sometime "later this year." If you're a Mac or Linux user, you might want to skip this altogether... But if you're interested in what looks to be the first usable version of Windows since 7, read on.
I loaded Windows 10 Preview on one of the laptops at work this week and made it available for people to play with. I can't vouch for its stability - though some bits may be a little flaky, most of it seems to work. This is just a technical preview, so don't be surprised if these comments are quickly rendered out of date as the updates are released and later when the Consumer Preview comes out.
Here are a few of the features I thought were cool:
• The Desktop is back and the kludgy "modern" UI relegated to the start menu.
• The start menu has all your software including tiles for your modern apps (or not, if you prefer)
• Modern apps can be dragged and re-sized at will
• Task View shows you all the apps and windows you have open
• Virtual desktop - this is a cool feature I haven't had a chance to play with yet
• "Tablet mode" makes it a little more like the current Windows 8.1 environment
• The Siri-like Cortana assistant is now available, but I think it may be a while before it's really useful.
I've only spent a few minutes with Windows 10 but, so far, it looks good - much better than 8/8.1.