Completely agree with you there, both the i8 and the i3 in my opinion look like electric vehicles you'd read about in a sci-fi novel, but neither are cool or sexy.
I don't think that the diversity of headlights is an exception to the rule of cars all looking the same, but rather one instance out of many of designers having far more freedom than they did historically, and so there's a lot more variation in what they produce.
Fewer and fewer people are buying sedans, but it's easy to pick a couple to contrast. Take the Ford Fusion and Taurus. The Fusion is all ovals, while the Taurus seems to be made out of chamfered blocks and trapezoids. Or consider the previous generation Hyundai Sonata. The current one is generic looking, I would agree, but the one sold from 2010-2014 looked like no other sedan that I've seen.
It looks a bit like a Mustang.
I felt the i8 was an £80k car with £30k of tech, whereas the Tesla was a £30k car with £80k tech.
As an unexpected bonus, my pregnant wife found it easier to get in and out the i8 compared to standard cars entries :)