A lot of the better beers like this that I have had tend to almost take on a degree of flavor, appearance and mouthfeel of fruit juice. No - not saying it is just like a glass of orange juice..... but, the best ones do come across as grapefruit, tangerine, peach, orange, etc..... Because they also use high percentages of flaked grains, the mouthfeel is full and silky to some degree.... like juice. The beers rely on lots of "tropical" type hops..... Citra, Galaxy, Mosaic, Amarillo, Simcoe, etc....
The beers tend not to be bitter, sharp, dry, crisp.... rather, the hops are rich and almost coat your mouth... kind of like after you take a swallow of orange juice.
To be honest, I think it is as good or better a descriptor than all kinds of beer terms....
You can't rip on "Juicy" being too vague because you don't know what "kind of juice" it is referring to and on the other hand think that "floral" or "earthy" are good descriptors. Floral? What kind of flower? A rose? A thistle? A marigold? A Carrion Flower?? Or Earthy.... well "what kind of earth?" Sand? Clay? Compost?
I mean we can get ridiculous about EVERY single beer descriptor that exists. I think people are going out of their way to find something to hate about this description. If you put a sierra nevada pale ale and a beer from Tree House in front of a thousand beer drinkers and had them pick which one to describe as "juicy" I think the overwhelming majority would pick Tree House. If you did the same with other beers and had them pick which one was "earthy" or "dank" or "floral" or some other arbitrary beer term.... I think the success rate would be lower. So, if it is something that a lot of people can identify with - it is a fairly good descriptor.