Lately I saw Chris Whitley live and he played some songs from his new album. A few covers ,i don't remember the name at this time. But he gave them a very personal touch. So that's the main thing if you cover a song.
There are many songs covered the wrong way, take All along the watchtower by instance, basically a very simple song, U2 called it three chords and the truth also adding some interesting lyrics about a red guitar, . This U2 version is one of the worst and that's the %^&%^%^& truth.
Only Jimmy Hendrix took this song to a high level mainly of his relaxed vocals and solo work.

Nowadays there are covers to see on TMF and MTV turned into the R&B thing. Mostly very taistless versions. Only the kids don't know the originals. When i try to make them listen to the original they are not interested.
Maybe one of these days Stairway to Heaven gets an R&B touch with a rapped solo, ha ha.