I've been a user of Yahoo Messenger 9 for months now and I loved the look, the feel and I've gotten used to most of its features but now since the release of YM 10, I'm starting to like the new version more. At first look, it doesn't differ much from YM 9 except for a few minor design tweaks. Upon signing in, you'll immediately see an option to choose the language you'll want your messenger to be in. I think this is the first thing that you'll notice with YM 10.
There's a new Yahoo Insider button at the bottom right of the menu bar. I've set my settings to default so that the Yahoo insider won't show as soon as I log in but for some users this is the first thing that they see. They've vamped it up as well. It now includes your Yahoo Mail, news, weather and Yahoo search.
You now have the option to directly click on an update and it automatically shows on the IM chat window of your friend so that he/she will know that you're commenting on his update.
You can now also get alerted by a "toast" popup window on the bottom left side of your taskbar everytime a contact makes a status update. You can disable this anytime in the preferences menu.
There are also new icons in the IM window. There are now specific buttons for video and voice calls plus an activity button with options to interact with your friends.
The major redesign that they did, although I haven't tried it out yet was for the video calls. I found out through the YM10 blog that you can now make a HQ video call with synched audio. Additional features include swapping video windows, positioning the windows side by side, muting the call and placing it on hold. The only catch is, your friend has to have YM10 also.