A few clarifications on K2 and the others :)
1. Both K2 & Zoo were launched back in 2009. Zoo is not new, but it has followed K2's lead and this shows in several aspects of its content creation process.
2. K2 did not lose any momentum when Joomla 3 was released. Way back in 2010 it was announced that K2 would not immediately jump to support Joomla 1.6 as it was full of bugs & problems. K2 was made available when Joomla 1.7 was launched, which resolved many Joomla related issues: http://www.joomlaworks.net/blog/item/156-why-were-not-porting-k2-to-joomla-16-the-life
That was actually the time Joomla lost momentum (and not K2), as many Joomla 1.5 owners did not see any real benefit jumping to 1.6 especially when that meant basically re-building your site.
3. K2 does extend the Joomla user (object). By default it enriches Joomla user profiles with stuff like an avatar, short CV, a site URL etc. And there's a K2 plugin that adds more fields to that (more personal details & social profiles of the user), which is drop dead simple to extend and add more fields: http://getk2.org/extend/extensions/73-user-extended-fields-for-k2
4. Zoo is presented as more flexible compared to K2 when it comes to constructing your content. This is not true. You may be able to move content elements blocks around in your Zoo item page (you know, have the date before the title and all), but you are limited to a flat style presentation of elements. That means, Zoo item elements will be stacked one on top of the other. There is no way to have e.g. 2 elements appear horizontally. There are no regions identified in the backend editing view of your Zoo item structure. So if you wanna do customization properly, you WILL end up editing code in the end. But if you are to edit code in Zoo, you'll quickly burst into tears... On the other hand, since there is no way of knowing all possible content element combinations in an item page, K2 was built with code editing in mind. That means K2 template overrides are not only dead simple to understand and edit as you wish, they are also very few (so you don't get lost) and very well documented compared to Joomla core content (dozens of template overrides!) & Zoo. Not only that, you can also create different sets of template overrides neatly packed in folders, which you can assign even to specific categories. And of course you can re-use these sets of template overrides to other sites. This also makes creating Joomla templates that support K2 a piece of cake. And this is why ALL template providers have embraced K2 so much, instead of any other CCK. In the end, it's more likely to find the template you need with K2 on it, instead of editing Zoo.
Of course there are many more benefits of using K2: from content settings (K2 category inheritance being a killer feature), to properly maintaining a category's layout no matter how you landed on that category page (unlike Joomla which still doesn't do it properly and you can easily end up with 2 or 3 different layouts for the same piece of content!), to performance (resized images, extended smart caching mechanism, 1 CSS & 1 JS file loaded only, optimized db scheme & queries reviewed & approved by Percona, the top MySQL experts worldwide), to ways of extending K2 with K2 Plugins (when extra fields are not enough - just see what people are build here http://getk2.org/extend/extensions
5. You mentioned that Zoo offers the "unique" option to hide stuff like comments on a per item basis. Flash news: K2 can do that for any content element in the item form.
6. The Joomla Magazine does not use native K2 comments, it uses CompoJoom's comments system which has a K2 plugin to replace K2 comments. That's how extensible K2 is - it even allows third-party developers to extend its core content functionality.
7. This is more of an observation and less a clarification. How many TOP Joomla websites do you know around the world that were NOT built using K2? Food for thought...
And to summarise things: K2 in nearing 2 million downloads. This is a very large number for an application that's essentially challenging Joomla's "CM" capabilities. Top sites across the world are built on top of K2's content features. Almost all Joomla template clubs provide some sort of K2 integration (all the top ones go one step further and even provide more than CSS customizations). There are more than 200 extensions integrating K2 exclusively in the Joomla Extensions Directory and almost 600 in total that interact with K2 one way or another! K2 has been audited by the Joomla core team and passed all tests for both code quality and security, before it was decided to power the Joomla Magazine's content. And finally, the coolest band on earth, the freaking Gorillaz, they use K2 on their site: http://gorillaz.com
K2 version 3 (the next big update) which will be available very soon is bound to redefine both content management in Joomla as well as HOW Joomla extensions should be built. We provide a small glimpse here: http://getk2.org/blog/1792-k2-v268-released-whats-coming-in-v270-a-v3-update