The term "mainstream" cruise line casts a wide net. From my multiple sailings with them, let's just say that Celebrity is a "higher tier" mainstream line compared to, for example, Carnival. Granted, all lines have their pros and cons. (Below list care of Cruise Critic)
List of Mainstream Cruise Lines
Carnival Cruise Line
Celebrity Cruises
Costa Cruises
Cunard Line
Disney Cruise Line
Holland America Line
MSC Cruises
Norwegian Cruise Line
Princess Cruises
Royal Caribbean International
Case in point, these images are from the casino on the Mardi Gras (thanks @IntrepidFromDC, only in service about 1.5 years. I don't think I've ever seen anything like this on Celebrity! They should REALLY be embarrassed having these out there for use...