Well the f/1.4 35mm, 58mm and 85mm have no play in the manual focus ring, and I find those rings easier to use. I suspect the f/1.4 24/35/58/85 are built to withstand rough use which they f/1.8's may not be quite as durable. My 24/1.4 and 20/1.8 unfortunately have this slight play in manual focusing (when reversing the direction of turn there is a "gap" during which the focus is not being adjusted even when the ring moves). I find there is a special "look" to the images from the nano-coated f/1.4 AF-S Nikkors, whether it is worth the price is anyone's personal decision but I do like the consistency and look, very well suited to the use of shallow depth of field in documentary photos and portraits. I guess they also fit into landscape photography for a shallow depth of field effect (not that you can't stop them down of course you can).
I suspect the main differences between the f/1.4 and f/1.8 Nikon lines is that the former is expected to interest a smaller group of customers with special needs, and the highest expectation of durability and a distinct look (well, it is arguable, but ...) to the images, whereas the f/1.8's are expected to appeal to the more price concious buyer who wants high quality images, chooses primes for portability, smaller size (than f/2.8 zooms) and is interested in best value. I personally find the 20/1.8 very sharp and high quality but something about the 24/1.4 makes the images look more "delightful" for lack of a better word. It is as if it wasn't quite meant to render reality in 2D but transform it into something slightly different. I think the higher price of the f/1.4's mostly reflects hand made elements and a small volume of lenses manufactured leading to higher research and development cost per lens. Also it may be partly due to cost of workers in Japan / China / Thailand. Of course the size of elements is greater also for the larger aperture variants.
Leica is a bit too expensive for my budget, and I like Nikon lenses and the image quality and viewfinders. I do appreciate each brand has their advantages and disadvantages. EVFs, however, are not for me, and if if I were somehow to buy a Leica it would be a rangefinder.