"you have to decide exactly what you want your 4K monitor to do"
No I don't. I can always choose not to use the tech until they get it right, and if they never do, eh.. oh well!
High input lag makes this a particularly poor choice. Input lag impacts every task, not just gaming. Forget it.
Gamers are really in an "interesting" place this year. You can't get a video card to drive UHD even with the newest chips, and buying a monitor is a minefield. Sure, you can do SLI to get to UHD, that'll get you most of the way there... except certain games (AC), and immediately after any game's release (Titanfall), and sometimes you'll need lower settings to accommodate VRAM issues (Evil Within). This of course bodes poorly for games to be released in the upcoming year if you're buying now. It's the wait for proper support that's really disappointing (usually good support, but look at Titanfall and CoD Ghosts as long waits).
On the monitor side, you can go to 1440p, and watch as your tech is outdated quickly (as 4K/UHD gets its act together...maybe) - and be permanently stuck with a resolution that doesn't scale 1:1 with 1080p (again, hope you're running good GPUs). In all monitor tech, you can get low response times, or great colors, or take a risk on a foreign vendor's product that MIGHT be tricked into doing both but will still have some blur/ghosting. You can get Variable Refresh tech that'll work with one brand of GPU but not the other. Lightboost/ULMB or 3D support is up for consideration, but can't be used with AS/GSync.
I can't help but think it's all a gigantic mess right now.