One thing that comes to mind is GSync. The technology synchronizes GPU frame rates with monitor refresh rates, and the result is most impressive. The only real obstacle to ownership has been cost, thanks mostly to the overhead of licensing through Nvidia. Most people who have this feature will say it's worth the cost (but of course, cost is subjective).
However, just after Gsync became very popular, another very similar technology was showing up, and has now been officially endorsed by Nvidia as well. Variable refresh rate (much like Gsync) is an intrinsic part of the DisplayPort 1.2a spec. AMD uses the same technology in their implementation, called "FreeSync". The difference is that the Nvidia Gsync requires a hardware module in the monitor (and the associated costs), where Freesync uses DisplayPort 1.2a protocols (software commands), thus not requiring the cost. Interestingly, even though the protocols are part of the DisplayPort specs, Freesync supposedly works with HDMI connections as well as DP (I haven't explicitly tried this). I've used both, own both types of monitors, and they both work. How well and to what extent can be much more subjective than simply whether they do work.
Nvidia has (finally) recognized the DisplayPort 1.2a protocol frame sync method, by testing and approving what they're now calling "Gsync Compatible" monitors. They don't use the hardware module that a true Gsync module uses to do this, and they aren't supported by as wide a range of Nvidia GPUs. While Gsync works on many cards all the way back into the 600 series, only the 10-series cards support "Gsync Compatible" monitors. The good news is it's an equally impressive technology, and it's cheaper than true Gsync (because the hardware and associated licensing aren't required). It's technically not as capable as true Gsync; for example the range of frame synchronization is less than with true Gsync, but as I said I own and have used both types of monitors and IMHO the Gsync Compatible setup is plenty impressive on it's own, especially considering the cost is generally much less than true Gsync.
There is also the AMD arrangement, FreeSync...although I do have some AMD GPUs and have tested them with a FreeSync monitor, I haven't spent a lot of time with it. That said, however, I'm sure if you're a fan of AMD graphics, FreeSync is definitely worth considering. It uses the same open-source protocols as 'Gsync Compatible" so I'd fully imagine it's impressive as well.
Another feature I've become very fond of after purchasing one myself, is an "UltraWide" 21:9 aspect ratio monitor. The extra width really adds to immersion by filling more of your normal field of vision, IMHO. Mine's a curved 34" 1080p 21:9, and even though it's Gsync, the cost for what is (currently around $500) is well worth it, I feel.
One final thought, IMHO: You sometimes see people bragging about 4k monitors. I would honestly avoid this...it might be great for TV - definitely a remarkable picture, without doubt. But when it comes to gaming, the extra cost associated with driving a 4k monitor at even close to a solid 60FPS (never mind anything beyond 60Hz) is still way on the ridiculous side. To me. If you have money to burn, good for you, but I still don't think it's worth it, and a lot of "competitive gamers" agree: The video is dated, but still applies, for the reasons it explains 4k Gaming Is Dumb
(And before anyone starts crying about it, that is the video's title as posted by the author, not me - and this guy/the site is fairly widely respected for their opinions, BTW).
The thing is, as the resolution increases, the number of pixels goes up dramatically, which means it takes a more and more powerful GPU to drive all that at a decent frame rate - this can easily run you up near a $1000(+) budget for a GPU, and that doesn't count the cost of the monitor itself. Also, most gamers by far strongly prefer the responsiveness of higher-refresh rate monitors, and just like with GPU and 4k resolution, it gets awfully expensive to buy high refresh rate, high resolution monitors. If you want additional premium features like a fast response time, a certain type of panel technology, or a curved screen...well...make sure you've got a ton of cash. And, in my experience, as well as all the gamers I've built systems for and many of the reputable sources online , it's just not worth what it costs...unless of course your objective is absolute bragging rights. And there's always plenty of that on the internet
Anyhow, that's my $0.02. Any questions welcome.