The rice is really important in sushi - actually, “sushi” means rice, not fish, and some types of sushi have no fish at all in them (mmm… plum rolls).
Preparation of sushi rice is one of those things where the most basic basics are simple but truly mastering the thing takes years, or even decades. So, in addition to the chef only purchasing the most excellent fish, he would likewise have to have the most exquisitely prepared rice. Likewise, all other ingredients would have to be the finest, like actual wasabi and not simply dyed horseradish, fresh vegetables, carefully prepared eggs, etc.
Also, some of the plain and simple appearing Japanese cultural things incorporate a really high level of precision. And a lot of sushi has to do with the presentation - the proper color and shape of dishes for serving maki and nigiri, ornamentation (in low-budget places that might be plastic representations of garnish, in truly high end establishments it’s elaborately cut/shaped/etc. leaves and stuff), and so on.
So truly high-end sushi involves very precisely prepared, very fine ingredients arrayed and presented on carefully chosen dishes/trays to complement the colors of the food, often with a very formal placing on the table. If the cook likes you, there can also be specially prepared items for your specific pleasure.
I went to a sushi place with a friend some years ago. After observing our choices - without us being aware of that observation - and our evident enjoyment the chef presented us with a tray that contained three specially made items: one for me, one for my friend, and one for us to share. The individual items were specific to our favored but differing tastes,and the shared one targeted those things we both liked. It was a very flattering thing to do for us, and how a sushi chef may distinguish himself from others. The two nigiri and the maki roll didn’t look particularly special, but they tasted wonderful.
Mind you, I am not some ultra-sophisticated sushi snob. I can happily eat “supermarket sushi” and enjoy it. But for a complete sushi experience there’s more than just the food, all the senses are supposed to be incorporated. It’s the difference between listening to opera on your iPod and actually attending a live opera production.