For simple, single flavor type juices, like fruits or plain aromatics, purity of flavor is important- no weird chemically aftertaste- and strength- in a single flavor juice I think it should be upfront and very clearly what it is. If there are flavors enhancing the main flavor, they should be in the background doing just that, enhancing and supporting, but not obvious.
For multiple flavor blends, nuance and balance. Every note needs to be there for a reason, complimenting the other flavors, with nothing jarringly ovewhelming or out of place. There's an art to it, and I really admire people who do a complex recipe well.
Above all else for me is that chemically aftertaste thing, which is more noticeable in complex flavors, I think- I can't stand it, and I can't vape it. Weirdly enough, the biggest offenders for this aftertaste seem to be the ones who advertise their juices as organic or all-natural. Steeping can help get rid of it, but for some juices, it never goes away.