I think the best vocals in trance are either ambiguous with possible hidden meanings;
"Sky Falls Down" and "Miracle" come to mind.
Or insightful, like "Gamesmaster"
or if they're going to be a bit more obvious, they need to work with the music synergistically, like "Going Wrong". It may be cheesy, but after a bad day, it's impossible to thump it out, as loud as possible, sing along, and not feel miles better afterwards.
Common vocal let downs;
i) badly sung
ii) the wrong sound of voice for the rest of the music
iii) the wrong lyrics... you can't (generally) put soppy love lyrics over a happy tune foir example,
iv) Lack of imagination with the vocal sound; it needs to add interest to the tun, not just mindlessly follow the tune. "Waiting" for example, the way the vocal goes up when the bass drops down, and vice versa helps lift both.
v) vocals added as an afterthought. The best music is written with a strong mental state/emotion. If the tune's to have a vocal, it needs to be done at the same time.