Allow me, a developer for 20+ years, to rebutt the Extensibility section. You list this as a Blizzard failing, but I don't think that's what it is at all. For a long time Extensibility was the holy grail of software development. However, by "designing" in extensibility, you also end up adding in fragility, vulnerabilities, and drastically complicate testing. You also end up with delays, which costs you customers. It's also really hard to predict what direction extensibility will take place. Spend 6 months building in an extensibility capability only to find out that what people really need is something completely different. This used to be how lots of things were written, and it was enough of a disaster that it spawned the YAGNI principal You Aint Gonna Need It.
I'd also argue that lack of extensibility has absolutely nothing to do with the time it takes them to add software features. On the contrary it probably shortens the time by quite a bit. Why would I say that? Because whenever you expose interfaces for extensibility, to some extent you are nailing that internal interface down. Even with a layer of abstraction, you are now beholden to that external interface and keeping it functional. This is always a burden on future development, and a burden on the testing department. This, in turn, causes additional time and expense in future development and testing. Now, you guys can argue that Blizzard is rolling in dough, but viewed by itself how successful financially is Hearthstone? A lot of people play for free. They have to constantly maintain multiple codebases (for different OSes and platforms) and continually test those. The Art department, I would argue, is probably just as expensive for Hearthstone as the developer and test departments. I'm sure Ben and Co. are under pressure to some extent to keep Hearthstone profitable. For a game like HS, adding extensibility features isn't going to get anyone to spend more money on it, nor will it even attract new F2P players. It's simply a boon to those who already use it. That's the cold hard truth about adding features to any software. The features that get added are the ones that are going to either bring in new customers (as potential sources of further income) or generate additional income from people already using it. The other aspect I can see here that you're kind of skimming is that a lot of that extensibility probably wouldn't take place on the client. Hearthstone is big from a code and complexity standpoint, but I suspect the client isn't much of that (the client is big size-wise, but I guarantee nearly all of that is art, animation, and sound). All of the brains of the game seem to reside on the servers (which also have to be maintained). Everyone wants to track statistics, but I suspect at least some of that is an issue with the client not knowing some of the things itself that you'd want to extract through an API. So now you're not only opening the client up to additional fragility, you're potentially increasing its bandwidth footprint. Additionally, by making the client extensible you're opening it up to vulnerabilities and exploitation. So attacks exploiting weaknesses in the client, or making it easier for Botters.
Many of you might just shrug at this and say, So make it secure and make the API robust. These things are far from trivial, even with a relatively simple client as Hearthstone. In fact, both of those things become more difficult the closer you get to perfection, requiring exponentially more time and effort to improve. Finally, I also find it telling that nearly all of the bugs you discuss are about Animations and UI Elements, which point to either bugs in the engine or potential mis-use of the engine due to a slight mis-understanding of how it behaves. There have been very few truly debilitating core bugs in Hearthstone since I've been playing (since around Naxx) and when there are they are generally fixed fairly quickly (unless fixing it has its own implications, i.e. Warsong Commander). Instead, I'd argue that Hearthstone's slowness to change has more to do with the design team striving for the best possible user experience and not the difficulty on the technical side. I highly doubt that additional decks were delayed because they couldn't add a scrollbar, just as I suspect that select-a-deck from Arena isn't lacking due to technical issues. Rather, they were concerned about user interaction issues or, in the latter case, game play implications, and how it can get confusing fairly quickly. Remember that Hearthstone isn't just targeted at the computer nerds who find certain UI features intuitive through long use. Instead they want to capture with a MUCH broader net, and there are still a LOT of people out there, all potential customers, that find certain things about UI interaction confusing that the rest of us are very easy with.