Soloists, I don't really care too much because I've probably heard the song a zillion different ways, anyway.
But, one of my greatest pet peeves is when a lead vocalist sings a song word-for-word or run-for-run as the original lead did it. Like Jonathan said, sometimes, it's just a matter of them not knowing the song well enough. And often, it's because they just don't have the ability to ad-lib very well, or the creativity or vocal skill to make the song their own.
At my church, we had a lead (and this chick could BLOW...) whose vocals were just ridiculously hot... but, she sang every single song as though she had memorized it line for line and run for run. The one that stands out the most is when we did James Hall's God Specializes a few years ago. You would've thought Tuneshia Crispell was in the mic herself... it sounded great, but it's so annoying, to me.