Recently I took it upon myself to see what people thought the greatest Hack and Slash game of all time was. Because really, that is my real genre of choice. But I was disappointed to see games such as Ninja Gaiden and Devil May Cry on those lists.
Don't get me wrong, those are great games, but I don't consider them Hack 'n Slash. Here is why: They are linear. They have a story to follow, it's ABOUT something, you are progressing towards something. There are puzzles, and boss battles.
I consider hack 'n slash like dynasty warriors, or devil kings. There is a story there but it's not nearly as important as killing everyone. Unlocking people and mounts, unlocking costumes, unlocking cutscenes. They are repetitive and if you've played one you've played them all, but the name of the genre screams "repetition!" hack and slash, rinse and repeat!