[pokerswami]

In additon to what's been mentioned, appropriate lighting and good air conditioning and heating.

Good quality 100% plastic playing cards, usually KEM or Copag, and keep them clean.

Disallow bad behavior by players toward other players and toward the staff. Allowing players to be belitted, berated or demeaned will definitely hurt attendance over time, especially of the types of well to do players you want to attract. Ditto for staff in relation to players and to other staff.

If it's underground, it's important to keep out people involved in any illegal drug trade. Those people attract law enforcement attention to the place even if nothing illegal other than poker is going on there. There is nothing more attractive to law enforcement and their penchant for seizing money and other assets than drug related persons. It's like an NFL team at a juicy buffet.