Quite a few, in fact. Here are the first that come to my mind :
No more critical finish. More or less replaced by Critical Edge, special moves that drain the Soul Gauge. Among which...
Guard impact is now a special. Performed through 4A+B+K, it will consume half a level of Soul Gauge.
Character customization has been decoupled from the game mechanics : Equipment no longer alters the character's stats. If you come across a guy in a silly costume, it's of his own will, not because he tried to max out Defense and HP.
Faster gameplay. Versus time limit can no longer be disabled. The maximum time that can be set is 60 seconds. Time Out should be a rare occurence nevertheless as most move sets have been drastically sped up, with attacks being sensibly more powerful than before.
Movesets revisited. Ivy's stances have been removed, Natsu, while a spiritual successor to Taki, plays differently... Everyone is faster, as I said before, and there's the addition of the Brave Edge moves.