Ill throw in my 2 cents here but I am by no means super experienced in this realm.
Amenities is not a huge deal for me. Generally I fly to go somewhere, while a nice interior is always a bonus Im fine as long as there is a bathroom and a receptionist to pay. Generally Im at the airport to tie up and then leave and go about my business. Some places to sit to wait out weather are nice but I by no means care if the couch is leather or 1970's velour as long as I can sit on it for a while.
I flight plan with foreflight so I wont comment on what you should have but in todays day and age I would consider a wifi connection in the FBO a must have since lots of people use tablet based planning now.
Fuel cost is always a consideration and you should try and remain competitive to your local areas as much as possible.
The key for me is a well laid out and clear website that lists prices for what ever you offer and other important information. Maybe an airport map with the location of the FBO clearly marked. A phone number to call for questions.
A solid relationship with a local taxi service is a bonus and a courtesy or rental car would be even better but I understand how this could be a runaway cost on some level.
Local knowledge is good if there are specific things that may be odd about the airspace and major airport issues that may arise.