Not trying to be snarky or pedantic: stuff that changes the actions you take.
For instance, a sleep tracker or 24hr HRM doesn’t really change what I do. An Ambit 3 might not be able to tell me my 24hr RHR, and a new watch can, but it doesn’t give me much more info than a “Huh, neat.” I’m not changing what I do to chase that number, it’s just reporting something that’s a byproduct of everything else I do. I already know how much sleep I get because I go to bed at X and wake up at Y. A colorful graph doesn’t change how tired I am or am not.
Same for training load. I understand the concept in theory, but who are these people who are adjusting training because of what the watch says? It’s cool to see the watch say “Peaking” or whatever, but this is likely a result of the plan you’re on. I guess I don’t see the Venn overlap between people who are spending 450-700 dollars on high end sports watches and yet aren’t on some sort of progressive plan. Again, I’m sure there are people who are, but big picture?
In terms of actionable data, basically all the standard metrics (pace/power/time/etc etc.)
This isn’t to say watches haven’t improved. Better battery live, better GPS accuracy (or not…), optical HR (when it works, or just as a rough estimate), these are all improvements to the experience for sure. I’m not here to advocate people go buy 5 year old watches. Moreso, just to say… I think a lot of these features are added in because you have to tout what’s “new,” and you can’t make pace, distance, and time ‘new.’ Just my 2c.