Originally, caffeine content was probably an important factor. There's a study that showed participants developed a preference for the taste of drinks that were

Originally, caffeine content was probably an important factor. There's a study that showed participants developed a preference for the taste of drinks that were accompanied with caffeine and theobromine. This preference became "highly significant" over the course of 6 tasting sessions: https://www.ncbi.nlm.nih.gov/pubmed/15772863 https://www.ncbi.nlm.nih.gov/pubmed/15772863 But consider that tea has been selectively bred for taste over the course of millennia, with different cultivars seeking out different ideals of taste, combined with a history of experimenting to find the tastiest methods of processing and preparation for specific cultivars of tea. If you're talking about tea bags commonly found in grocery stores, then yes, it could be any drinkable plant with added caffeine, and nobody would notice. But there is definitely something special about the taste of a good sencha or long jing.