The shared move lists are a big boon. Almost all of the characters share the same input list (with a few tweaks and exceptions here and there) which means you can switch between characters right from the off (providing you can do a 2xQCF movement for specials etc) - the same deal as with Last Blade 2. This means that the muscle memory threshold is greatly lowered, meaning you can move on to tactical play from an early stage.
It's a simple game to learn, with straightforward supers and specials and, by cutting out a lot of the fussiness of the earlier Fatal Fury titles, results in a much more focused and streamlined experience.
It was one of the last games from SNK proper (before the Playmore take-over), so the animation and sprite work on both characters and backgrounds comes from an art team at the height of their game. Likewise the character roster is awesome, without any weak links.
The game has its detractors for some of the above reasons, but I'm a true believer. I don't especially see the 3S comparison though.