The win with a ~200dpi 24" 4k will be OS X-style Retina upscaling, not simply screen real estate.
(2) Being too large is relative, my 30" 2560x1600 quickly became normal sized. For the additional pixels the additional inches of a 39" seems about right.
(iii) HDMI at 30Hz is just fine for anything but gaming. I used to have an IBM T221 that also ran at 30Hz and it was no problem at all for text and video. Some people expect there will be mouse lag, there wasn't.
The difference is staggering. 30Hz is terrible, and the lag when moving the mouse, dragging a window, or scrolling text is perceptible and annoying.
I used the T221 at 30Hz in a multi-monitor system with the other two monitors at 60Hz. I could drag a window so it straddled the 30Hz monitor and a 60Hz monitor. The DPI was severely mismatched so it wasn't very useful to straddle like that, but movement across the monitors was not obnoxious, not terribly fluid, but not annoying either.
I'm sure different people have different tolerances for latency, but I tend to think my tolerance is pretty low.
2) 39" is not way too large. I want 50" on my desktop.
iii) Yeah, HDMI 1.4 is limiting, but 30Hz makes it still usable for non-gaming. DisplayPort? I guess I won't balk at it, but just give me HDMI 2.
I am critical of the monitor for other reasons: a) it doesn't power up on DPMI on and has a splash screen, 2) it has a semi-glossy screen and professional monitors should be matte, and iii) it's HDMI 1.4.