Hollow Knight, so far, is like Metroid (the first NES one) with a dash of Dark Souls.
New areas have no automatically updating map, like most Metroidvania games. There is no experience point system. Your abilities only increase upon finding or purchasing items. All items so far are really expensive and hard to afford, especially if you lose all of your geo while trying to recover your shade from where you died.
There are a bunch of enemies that will come out of nowhere, sometimes if you are trying to escape one area, backtracking, an enemy that you thought you killed will pop up and end you.
The graphics and music are haunting, the environments look engaging, but are also very threatening.