One of the biggest new features is video recording. The webOS can now record video whenever you want, which is of course a feature that should've been there from the get-go. The feature might be late, but luckily, it has some very nice touches that other platforms don't offer: you can edit the video right on your device, and you can upload it to YouTube or Facebook straight from within the recording application. Of course, you can also share the video
Another new feature - not listed in Palm's changelog - is support for Flash 10.1 from Adobe. Flash 10.1 for the webOS is still in beta, and the beta package is expected to hit the App Catalog any moment now. This is obviously beyond Palm's control - it all depends on when Adobe is ready.
Of course, this release also promises to deliver performance and battery life improvements. An informal perusal through user forums confirms webOS feels snappier after applying the 1.4 update, but this doesn't say a whole lot, yet, of course. As for battery life improvements, which are much-needed, the changelog notes that in 1.4, WiFi will be turned off as soon as the screen is turned off. This alone should save a few percentages of battery life per day.
The update has already hit pretty much every carrier, so chances are that if you carry a webOS device, you already have the update.