Can't sleep so I've decided to make a list of the notable improvements (no particular order). There are obviously other things I've left out, I'm just sharing what I feel actually change the overall experience for me.
Touch ID
As ridiculous as this sounds, it's now instant, and that saves a fraction of a second when unlocking, and as often as we tend to unlock, that's a lot of time saved over the life of the phone. Even if you aren't spending that extra time saving the World, or eating bacon, it's just a better experience. It also seems to fail less often (happened once in the two days I have had he phone and it usually happens 1/10 times) and is more accurate with moisture (i.e. At the gym)
Safari Tabs
Whether you care or not, it's a very noticeable improvement. I'm sure there is a limit but I didn't hit it. I just couldn't get tabs to reload. With the previous model this was easy. I also very frequently got the "error occurred page reloading" on the previous devices. Not so with the 6s plus. In the long run this saves data but also s ton of time. If you're actually trying to get something done and not just juggling buzzfeed articles you are saving yourself actual seconds which quickly add up to minutes. Of course, there is also the aspect of frustration, which has been completely eliminated.
Load Times
For me this is only really noticeable in a side by side. Pages load more quickly. Apps load more quickly. Phone boots more quickly. Everything is just slightly faster.
3D Touch
This is one of those things that I fully admit I couldn't understand why this is so different from a long press until I used it. It's actually remarkably unique (though I don't feel it's all that intuitive so there is a learning curve) and I do see this saving time and improving productivity. Among the things you can do with 3D touch is select text (to copy and/or paste), engage the app switcher, take a look at links (such as those on this forum) without jumping directly to that page (and then decide if you want to go ahead and Juno to that page), shortcuts to actions in an app to do start tasks before you've even loaded the app, and probably a lot more as devs add functionality to their own apps.
Despite all of this, the thing that brings me back is performance. It's possible I was abused by my 6 plus too long and I forgot what true love was. That device was just a dog and, especially now that I have something that works beautifully, I regret keeping it, because it just wasn't as good as it should have been. I absolutely did waste my money, but it wasn't in thisbhesr's upgrade. It was last year's. Live and learn I guess. I need to make sure not to settle. And I really KNOW that's not what I'm doing with this device.