Just started playing. It's surprisingly fun. May be a little difficult to play if you don't understand moonspeak though.
Key points to note:
Main purpose - Collect "Miasma" point to release the demon from his seal. This can be done by interacting with the yellow circles on the ground, or by performing H actions.
Action points - Performing tasks, e.g. collecting Miasma, working at the convenience store, etc. requires action points. Your action points reset each day. You can also restore some points by visiting the bath house (max once per day) and eating food purchased from the rat familiar in your house. (The demon will summon it when you speak to him)
Notoriety - See Sangoku's post about. Certain tasks will increase your notoriety. High notoriety will unlock some events, but also lead to bad ending.
Seal world - To enter the seal world, speak to the demon. The blue flames can be extinguished using Miasma points to undo to the seal. You can also collect "Magical" points here by walking into the black stuff.
Talent world - So here's the fun part. Interact with the bed, and one of the option will allow you to enter the "talent" world, which looks like a house with many copies of the MC. Ignore them (they are useless) and go to the top. Here you can spend the "Magical" points you gain from the Seal World to unlock "Talents", e.g. ability to attract molestors, prostitution, etc.
