Keep in mind that I don't own any of these cars (or any car). I am not a car guy. I have spent many hours in first-generation Leafs and a couple in Leaf 2 & Model 3, mostly through my local micro-rental service. You really should try this out for yourself :)

Not being a car guy, it's hard to explain, sort of like the difference between Android and iOS, or even more uncannily, between iPhone 7 and iPhone X, which shouldn't feel so different but do. It's like there's many small details that are below the threshold of perception which come together to make it an overall pleasurable experience.

The Leaf is fun to drive. The Leaf 2 is also fun, both feel like completely ordinary cars. Perfectly good driving experience. Preferable to gasoline cars, I would say. But the Tesla 3 is even more fun to drive, and feels like a more luxurious and relaxed experience, with many annoyances that you didn't know you cared about, removed.

Just to list some that are obvious: Nissan are terrible at software and UX. The screen reacts slowly, with low FPS, does counterintuitive things and is confusing. The gear indicator light cannot be read in direct sunlight. The motor makes a high-frequency sound when accelerating, even from a standstill. The charge lid must be closed manually if you open it by mistake. The instrument panel is cluttered and unintentionally visually stressful, in the way almost all cars' are. Handling is subtly less pleasant in all conditions. Acceleration is not as powerful, and somehow feels less controlled and more jerky, even at low speeds.

Contrasting with the Tesla: Regenerative breaking when letting up the gas feels more subtle. Suspension feels less wobbly. The accelerator doesn't cause a jerking feeling unless you want it to. Can get you up to the required speed faster than you'll know how to make use of, unless you've deliberately practiced that kind of performance driving. The screen software is more intuitive. Has high FPS, reacts instantly. Visually much more peaceful, minimalistic, beautiful. Ditto for the interior design. Speed indicator manages to be at the exact correct spot in your field of view, somehow encouraging you to look out the front window more and keep attention on the road. The panoramic glass roof is just so peaceful if you stop somewhere to enjoy the scenery.

There's stuff to be annoyed at with the Tesla too, there's room for improvement many places. The collision avoidance warnings in particular felt too loose to me, and the sometimes-inconsistent placement and interpretation of the traffic around you on the display doesn't inspire confidence wrt. self-driving. (Although, the fact that it's there at all is pretty cool though, it's fascinating to see the streaming output of a neural network vision system in a real product. It's obvious that Tesla is building this system from real-world usage data and the progress will be fascinating to follow).

Overall, it's a much more pleasant experience, IMHO. I recommend you get a couple of hours to test-drive and see if you agree, or if I'm just caught up with a weird case of the fanboyism.

pi-rat explained my comments about charging while on the road better than I could. The key point is terrible and inconsistent user experience among other vendors. And I've seen examples of people plain being unable to charge at the single charger that's available. Not a fun thing to worry about when 40 miles from home at low battery.