1. Most importantly, the ability of the keyboard to 'backspace' and suggest autocorrected words are BACK. It massively improves the typing experience on the already excellent stock keyboard. Previously this function was removed in Sense 5.5, and I had to delete an entire typo word and retype again.
2. Sense 6 use a color code across most areas, and you can change the color in the setting. A same color is used in blinkfeed, the fast toggle menu, on the keyboard itself and in the loading page bar of HTC browser. The UI continues to be smooth and fast.
3. Power usage now shows a much more accurate breakdown of power use by various apps- you can find exactly the app that you use the most to be at the top of power use list. It was almost 'useless' in sense 5.5. And great news, so far I haven't noticed any quicker battery drain than Android 4.3. The drainage from 100% - 90% is no longer as fast as it was previously.
4. This phone acquires a new ability - extreme power saving mode - which could save your day when your battery runs low. I have yet the test how well it helps with battery life in real life. Definitely will come back when I use the phone more.
Some let downs though:
1. Blinkfeed is still not quite there for my need.
2. Icons and interface in the original HTC apps like music got a flatter look (I don't know why it became a trend in all major mobile software platforms)
Lastly - Good job , HTC.