Probably the general glitz, the vibe, and good dining...DH's former boss and his wife LOVE Vegas. They go at least once a year, usually more. They don't gamble (I think his wife plays $20 in a slot machine each trip and that's it) and don't go explore the national parks at all. They occassionally go to shows and such but that's about it. It is unclear to me exactly what the draw is for them but they wonder the same thing about us going to WDW so to each their own.
Kind of like what the Boardwalk used to be like tbh.... Though I'm not a big Vegas guy, I could imagine just wanting to do a trip at the BW where you sometimes stumble into EPCOT or the Yachtsman...