I voted Aircraft as well, like Javier said it's flightsim. allthough i think (airport)scenery, mesh and landclass are important, i actually only fly payware (PMDG738,744(F), LEVELD763, PSS772, WILCO-FT A318,320, FLIGHT1 ATR72-500, DIGITAL AVIATION FOKKER 70/100, DREAMFLEET B58).
But what is FS without add-ons like Radar Contact or Active Sky or FS2crew or Navigraph (with every month a new AIRAC).
To make this world a prettier place i've installed FSGlobal X 2008.
And last but certenly not least: AI traffic. wich is freeware. Project AI for FS9 and World of AI for FSX.
So in my opinion there's more than one kind of add-on to make FS attractive.