4 reasons for me not to buy one yet
1) Price. Come down to ~$2000 for a 50 inch and we will talk
2) Standards. As I would be using this primarily as a monitor and not a TV I really need some standards to follow. Not just the HDMI issue, but all of the screens on the market today, when hooked up to a computer, act as 2 independent displays. My understanding is that some are interlaced, some are top/bottom, some are left/right, and some don't work at all. It needs to be a single unified display capable of at least 60hz that runs on a single cable.
3) GPU hardware needs to catch up a bit. The bulk of my content is software instead of videos, and all of that is ready for 4K glory... but my single GTX570 is not. Even a few Titans can't push those kinds of frames yet. In a few years it won't be a problem... but it is not ready for today.
4) Video Content. As someone else said, often 1080p is not done right yet by many distributors. That is not to say that 4K will not be an improvement... but things like contrast, refresh, frame rate (repeated or unique), and audio quality are more important than moving from 1080p to 4K. Thankfully 4K is the last practical standard for home use, so perhaps in 10 years we will see other things improve rather than sheer resolution.
All of that said, I still really want one. Editing 1080p video would be a much better process, multi tasking would be so much easier, games can finally get rid of AA lines with very little filtering, and pictures can be viewed much closer to native resolution. Lots of things open up with this screen size, and I hope to have money to buy one once they are affordable and standardized, but I fully understand that this is 'new' technology, and it is going to be another 5 years or so before it is really ready for prime time.