Personally, I don't like many of the newer hot tubs that are out there because they're too molded with too many sharp ridges and contours and I don't like how they force you to have to sit in a specific place and in a certain way. You can't sit an inch over in any direction from where they want you to sit because the tub is so highly molded, you can only sit in the exact spots and exact positions they want you to sit in. I also don't like that they have a zillion jets in them. Jets aren't that important to me and I don't use them much so I prefer a tub with fewer of them, not more.
I like a tub with smooth rounded contours and lots of open seating so you can sit wherever you want and in whatever position you want. That's why I went out of my way to find and purchase an old early to mid 90s Hot Spring Grandee and I'll try to keep that tub going forever. It helps that I'm quite handy and know their system quite well so I can troubleshoot and fix any problem that comes up with the control system or plumbing. To help keep the tub going I continually scour the classifieds to pick up parts I can use from people who are parting out their old tubs. With a collection of spare heaters, jet pumps and control systems that I've picked up for cheap, I'm entirely self-sufficient and can keep my old tub going for a long-long time at very low cost.
If I was looking at a new tub there are a few features I'd want it to have. One is that it uses a dedicated small circulation pump that runs 24/7 rather than try to make one of the jet pumps run at a very slow speed and circulate at certain intervals. Running a small pump motor that doesn't draw much current 24/7 is more energy efficient than trying to make a large electric motor that isn't designed to run at very low RPM do exactly that, even if it's just intermittently. Also, a little circ pump motor that isn't working hard and runs constantly will probably far outlast a jet pump motor that's being forced to run a lot slower than is optimal for the motor but I'm just speculating here.
Another feature that I would want is for the inside of the cavity between the tub and surround to be fully filled with injected foam. I live in a place where it can be very cold during the winter so I prefer a tub that is as well insulated as can be. Surrounding all of the jet lines with foam holds them in place rigidly and prevents them from moving and expanding from the pressure surge when the jet pump gets turned on. I believe that over time, lines that are able to jiggle and expand will start to fatigue at stress points and will eventually start to leak where fatigue cracks form. I can't prove this happens but as an engineer that is something I would expect to happen. As with most things in life, having a tub that's fully injected by foam like a Hot Spring comes with a trade-off. The trade-off is that if you ever freeze a line or a line starts to leak for some other reason, it is much harder and much more work to find the leak(s) and repair it/them because you have to dig out a ton of foam to get at them.
As for all the other features like waterfalls, light displays, sound systems, etc., that you can get with tubs nowadays., I wouldn't want any of them. More stuff to have to fix when it goes wrong and I wouldn't use those features anyways. Call me boring but I pretty much just like to sit out in my tub, soak in the wonderful hot water and enjoy the quiet and darkness as I look up at the night sky. Once in awhile, I might fire up the jets to work on a sore muscle but most of the time I never use them other than to circulate chemicals. That's why I don't need or want a tub with 140 jets. My big ol' Grandee has about 14 jets and that's plenty for me. You may feel differently about that though.
One new feature/capability that I might like to have is WiFi control over my tub's control system so I can do things remotely like adjust the temperature or operate the jet pumps. That's a new feature that could be pretty handy.