So I got the new Fire TV Gaming Edition a few days ago! I'm not a video or audiophile (no 4K TV or fancy receiver here) so I can't comment on those features. From what I've read there is a problem with the device and Dolby Digital 5.1 however Amazon said they'll have a firmware update next week that'll remedy that problem. The UI is exactly the same as before and navigation hasn't changed much. What has changed is the addition of Alexa and (owning an Echo) I find it nifty to be able to use some of those features on the Fire TV. The microSD card slot is a very welcome addition as now I can expand the memory VIA microSD while connecting an additional accessory to the USB (right now I have a USB receiver for a mouse and keyboard). General performance is noticeably snappier; launching applications, loading inside applications, and streaming video/audio is definitely faster.
Gaming performance has increased significantly; benchmarks show about double the performance in this respect. A good example is GTA Vice City. On the old Fire TV I was only about to put the settings around medium and even then the FPS took noticeable dips. On the new Fire TV I'm able to max out everything and the FPS seems to stay consistently at 30 or above. In addition to the increased performance the gaming controller is also much improved. The old one wasn't amazing...but it wasn't bad either. The build of the new controller feels more solid, sticks feel a tad bit tighter, dpad seems to not have as much resistance, and the contour of it seems better fitted for the hand. The controller also adopts a Xbox One style battery compartment with it being flush with the back of the controller and the headset jack for private listening is a welcome addition!
ADB works exactly as it did with the old Fire and I was able to sideload Firestarter (new update works great with the new Fire TV) and Kodi easily. Speaking of Kodi, performance was great and launching, navigating, and streaming video/audio seems a bit snappier. The audio level is much lower though and I find myself maxing out my TV volume just to get to a comfortable listening level. This only happens with Kodi though and I don't encounter this low audio level when watching/listening in other apps.
There are a few quirks I've run into with the new Fire TV. Right now I'm using my old Fire TV remote and there's noticeable lag between the button presses and the action on the screen. I don't experience this lag when using the game controller and I actually had another old Fire TV remote that had an older firmware on it and it didn't experience this lag either. Another quirk is that when you plug and/or unplug a USB flash drive it seems to make the apps on your microSD card inaccessible. When this happens you have to remount your microSD card to get access to those apps again. The low audio level mentioned above is another quirk.
So what's my verdict? If you're looking for a good media streamer then definitely consider the new Fire TV. There are some quirks but the device just launched and Amazon seems to be listening. The fact that they're releasing a new firmware to remedy the Dolby Digital issue shows that they're taking these issues seriously. If you're completely happy with your old Fire TV then there's not much new here...unless you absolutely have to have 4K and/or Alexa. If you do want those things though and also want increased gaming capabilities then definitely consider upgrading to the new Fire TV. There are many other options out there though (Apple TV, Roku, Shield TV, Nexus Player, Chromecast) and if you're not keen on the Amazon ecosystem then it's probably better to give more weight to these other options as the Fire TV is heavily geared towards Amazon content.
EDIT: Actually, reading on Reddit it seems some people are already getting the updated firmware with the Dolby fix.
EDIT 2: According to Amazon Support...
"Thank you for the feedback. By this coming Monday, October 12th, you will receive an automatic software update that enables Dolby Digital Plus for customers who have a Dolby Digital Plus system (including support for Netflix and Amazon Video). An update for older Dolby Digital systems (not Dolby Digital Plus) will be distributed on November 30th."