The Showtime apps don't support Siri yet. Siri support is limited to just a few core apps at this point, but Apple's opening an API for other apps.
Neither does Netflix. All of the apps are very disappointing, especially with regards to their layout and design - it's all just a massive wall of videos on a black background, with some (rather small, hard to read) headings. In Netflix, the header is just ridiculously large and will make your massive 60"+ HDTV feel cramped.
Flickr is another particularly poor App. I suspect it uses a built-in photo gallery view controller, but you can't see details about any individual pictures (only their creators with a horribly unintuitive long-press) and it doesn't even bother to preload the next picture as you're browsing through - this isn't a mobile device, and if you're flicking through photos you probably don't care if they load one more than is strictly, absolutely necessary. Even on a limited connection you're not going to count each individual photo you browsed through and moan that it loaded another one for the sake of smooth browsing.
YouTube is a total joke.
Anyway, those are third-party apps. The developers (no matter how big these companies are and how many developers they can throw at it) have only had the SDK for a short time. I'm disappointed with the day-one state of the AppleTV, and really hope it gets some traction so these Apps get significantly improved.
The AppleTV itself is not that solid, either. Apple doesn't have any excuses - they've had all the time in the world to work on this.
Siri is just totally broken for me. Even the example questions such as "how tall is LeBron James?" and "Did Liverpool win?" come back with "I don't know how tall LeBron James is" and so on. Not only that, but I use a UK AppleID for iTunes purchases but my Netflix account is from Germany (which is where I am), so Siri gives me totally wrong information about what is available on Netflix or not. I have my music on the device via iTunes Match, but Siri doesn't seem able to play anything from there (couldn't find any Bob Marley? Look again, Siri).
I've had wildly inconsistent experiences downloading the additional content bundles for Asphalt 8. The initial package was taking hours on my 100Mbit connection. Sometimes levels take a few seconds, sometimes half an hour or more.
Oh, I've had quite a few crashes, too. I was just watching something on Netflix, paused it for a few minutes and the device crashed and went back to the home screen.
I've found there's quite a bit of lag with the controller. Playing Crossy Road, I definitely felt there was a delay between when I pressed and when the character started moving (when pressing the touchpad down, so not related to sensitivity setting).
The controller itself is quite difficult when navigating around menus. The worst are those modal 'Do Something'/Cancel screens. The focus swings so wildly between the two options I almost always hit 'Cancel' by mistake. I've become really cautious whenever one of those pops up now, and I had to be very careful and deliberate with my motions; even then it isn't easy. Oh, and yes, I've set sensitivity to 'slow'.
Photos doesn't work with iCloud Photo Library. I have my entire photo library stored on Apple's servers and the only things my AppleTV's Photos app can show me are some stupid DubSmash videos somebody shared via iCloud (probably by mistake) a long time ago...
The Music app itself is a bit of a joke. It's just a big long list; seriously? Does Apple want us to all get RSIs swiping our way down those things? Also, the 'Artists' view will sort by 'Album artist' but group by 'Artist', so that list isn't even in alphabetical order! You can't really identify where any artist is going to be; it's a nightmare!
Yeah, not much to say. I'll forgive the crashes due to it being a 1.0, but not the Mad Hatter's Music App or lack of iCloud Photo Library. Apple said their Siri API wasn't quite ready yet, which makes me hope that Siri's Netflix access uses some temporary workaround solution and I'll be able to connect it to the correct region once it becomes something any service can plug in to.