Random observations from listening to a bunch of popular Vocaloid songs:
Japanese songs from producers with medium experience levels often have unusually mixed vocals that sit dryly on top of the instrumental. I think this is probably just the result of producers struggling to do everything. The fact that many of these songs are successful regardless goes to show that a song can still be widely enjoyed even if the mixing is a little crude.
Songs from English speakers also have issues with vocal mixing, but for different reasons. The most common problem I hear is lyrics that are too quiet. This seems to have a bigger impact on the song's popularity: we instinctively try and focus on vocals, straining our ears when they're hard to hear. This makes the listening experience tiring.
In general, the biggest thing that separates a very popular producer from one with medium popularity is their ability to write lyrics with naturalistic rhythms, like iambic pentameter, that fit the rhythm of the instrumental. It sounds like a small thing, but it goes a huge way to making even the most robotic vocal synth accessible to any audience. A Vocaloid2 singing a vocal melody with natural stress patterns sounds less jarring than a SynthV singing lyrics with an unnatural stress pattern.
English speaking producers sometimes make the mistake of transferring the rhythm of their favourite Japanese song onto their own, creating the strange effect of English stanzas sung with a rhythm meant for a different language. Interestingly, this makes the song sound like it was translated, and is a sound that's basically unique to our community.
may all your vocal synth wishes come true
