Critics have complained that YouTube's 4K capabilities aren't "true" 4K and that while the resolution may, in fact, be 4,096 by 2,304 pixels (over four times the resolution of 1080p) the bit rate and compression makes YouTube's 4K look worse than standard 1080p via Blu-ray. In fact, on YouTube's own blog about Friday's announcement, several users have complained about choppy playback, invasive compression artifacts and more, citing that YouTube's own 720p playback looks superior to their 4K offerings at this time. YouTube does state that in order to view 4K content properly on their site, users will need an "ultra-fast-high-speed-broadband" connection, not to mention a true 4K capable display among other things.
Still no one, not even YouTube, is claiming that 4K is going to become the standard overnight just because the world's largest online video service offers it as a feature. It's just a first step towards weaning consumers off of physical media and bringing them that much closer to an all-digital, high-resolution downloadable future.