The plugin update and installation process has been greatly improved in WordPress 4.2. You are no longer redirected to another page during the update, it’s now done via Ajax inline making the process much more enjoyable. Hopefully this leads to more people running updates on plugins.
New Emoji Support
WordPress will now have built in support for emoji icons. This may not seem like a big feature however it allows for support of Han characters (Chinese, Japanese, Koren) out of the box. This opens up additional markets that WordPress can expand to much easier with just native support.
The theme customizer will now support on the fly theme switching. This will allow the user to preview the theme and configure it before it’s activated live on the website.
This very under utilized feature is getting some love this time around. If you don’t know what Press This does it allows you to have a bookmarklet in your browser that you can quickly click on and quote any article online within your WordPress installation. There is some changes which allow you to now preview the content and additional settings are available.