To me it fixed an important part of the phone which was bluetooth streaming. The quality on the highs was miserable, now it's great. I even ordered a pair of jaybird bluebuds x since I was so pleased with the 4.3's update quality. Other than that, the dreaded menu button is gone thanks to a nifty and frankly superior implementation of the menu and google now button. Now you long press home for menu, and that is good since you can go back into the app if you mispress. Unlike the back button. Great smoothness, overall improvement. Also, the purple tint has been reduced.
And finally, Bluetooth low energy was activated on the 4.3 update I believe, or 4.2. The thing is that allows for use of the pebble, for example and other exercise peripherals with minimal battery drain. Look up the changes in android per se, since the changes HTC made are minimal. I for once think most if not all of the flaws are gone by now. Now I don't care about future updates, and may avoid them to be honest, since I'm not looking forward to anything else.