WordPress 4.4. had gone RC and while I am far from being a typical wordpress user, so maybe my opinion as a user doesn’t count much, it seems like it has no improvement in anything related to content production and consumption.
The REST API infrastructure is something that in the realm of big organizations. I just can’t see anyone fully separating client and server sides and connecting them with only API request. This is not efficient and will hurt SEO, and while sites that are more of an application then content (think google docs) do not care about SEO and the performance hit they will be hit with is just nothing to worry about when considering the advantages in software development practices that the separation gives you (good JS developers are easier to find then good PHP ones as almost by definition there are more of them). But for anyone else? The infrastructure can be useful in cutting development time of similar feature but even that is done by very few people.
WordPress as an oEmbed provider is a nice feature but not practical. The problem with oEmbed in general is that you need to trust the source of the embeds and there is just no reason for anyone to trust someone he doesn’t know. This feature can be useful for people that have several sites or in a network, but for people that have one site there is no real advantage of embedding over using some sort of a shortcode that justifies the performance hit that will come from embedding content in an iframe.
Responsive images, in a world in which with everyday mobile bandwidth becomes cheaper and mobile CPUs become stronger, serving an image which is too big is just a not very significant problem, and again people that just post an image from time to time and are not photo bloggers are unlikely to feel the difference. And this is actually the small problem with the feature, the bigger one is that there is no user control on which images are used as alternatives and in theory you might end serving some cropped images and some resized images as alternatives to each other, something that they are obviously not.
The only feature that does make me excited is the taxonomy metadata which will let developers write code which do not feel like a total hack when trying to add features to taxonomies.
Overall, almost nothing to get very excited about, but also nothing to really hate. I think that is the exact definition of the word “meh” :).