For me, Di2 shines on rolling terrain and on fast group rides. I shift a lot, looking for the best cadence. On fast rides and on rolling hills I shift even more often. I might shift for just a couple of pedal strokes, then shift again (and then again!) The Di2 is fast and just like a mouse click, I even use my ring finger at times. And I'll shift the chainring even for a small hill -- on a group ride this summer, we turned a corner and hit an unexpected short, steep climb -- the Di2 riders were all sitting and spinning up after some quick shifting, the others were grinding up in their flat road gearing.

I also have a gravel / all-day-ride bike that's usually ridden at an easier pace. I don't really miss having Di2 on that bike.

Larger tires!
When I was shopping for the "last road bike I'll ever need" (uh, 3 bikes ago...) in 2005, road bikes were using 23mm tires, and could fit 25mm for "comfort" riders. Now, 25mm are common, and 28mm are very nice if the roads are rough at all. I'm running 29mm equivalent -- my rims are wider than usual. At 170 pounds, I use 65 psi front and 80 psi rear. It's a smooth ride and still is fast. I think the larger, supple tires at appropriate pressures are way better at handing road vibrations than the frame does.

And newer bike can use wider gearing. I really like my 11-32 cassette on the hills.