Agreed. The OP asked for gimmicks that don't really affect the sound that DO work. Compressors, mute switches, tuner outs, etc are quite useful features, not gimmicks. The Trace Elliot UV light is a brilliant gimmick that did work and I dont know of any other manufacturer that has ever copied it.
From a 100 yards away on a dark stage, if the amp was glowing green, you knew it was a Trace Elliot.