Overly repetitive lyrics, especially when there is no change in pitch or timing. It's as if the writer/performer couldn't think of anything creative, so they just repeat the same short sentence, sometimes repeating just one word three to five times in a row for each run of the chorus, padding the length of the song up to the minimum for suitable radio play.
Also, like many here. I don't really care for the over use of particular effects, like auto-tune. Though i do believe it has its place as a method of artistry in music. It's a tool, and like any tool there is a right and wrong way to use it.
Same with vox screaming and growling in doom, death, and viking metal. It has it's place, but can be (and is often) very overused.
These things should be additions to add to the vocal lead, not necessarily be the entirety of the vocal lead.