Most people already look for the obvious ones (resolution, refresh rate, panel type, black levels, color accuracy) so I wouldn't actually call those underrated -- they are highly indicative of what you'll actually get.
It's not on the list, but I'd say the finish (matte, semi-glossy, glossy) is probably the most underrated spec, as it's not commonly advertised, and consumers usually have to dig for it.
Most underrated spec been looking at lately is Brightness for quality of the panel itself. Low quality panels are usually 250 nits. I won't go below 350 now and try and find 400 or better so i can actually do HDR.
I voted for this. I'd say that it is the most important feature that is often left off of spec sheets (If it isn't listed, it's probably TN). But I haven't really done any monitor shopping in a few years, and I think IPS is much more common than it used to be, so maybe it's not as much of an issue now?
Most overrated for me would probably be pixel response times. Sample-and-hold blur will usually be a much higher contributor to motion blur than pixel response times.
Static contrast. This number is the honest number of a monitor’s actual contrast ratio. Dynamic contrast ratios are mostly made up. Static is not. Now not all monitors hit the static contrast, but it gives you a good idea of what you’re getting. Whereas dynamic contrast ratio is basically the biggest number they can print on the box and is meaningless.
I don't think a single one those indicates anything. Its always a combination of features that determines greatness. And even then, it can have all the best features and still be terrible if they're poorly implemented.