Then there are songs that are covers, for the most part i am all for covers and remixes, but sometimes its just a way to be unoriginal. A good example of a remix is Sean Kingston "Beautiful Girl", his melody is a stronger force in the song than the "stand by me rythm", its different and its catchy in its own right. I've heard some just unoriginal stuff though like "Right Round", which really just relies on the original melody from "you spin me round". Its not catchy on its own merit, They just changed the lyrics and tried to pass it off as best they could as their own. Recently i heard a song on the radio that sound just like Justin Timberlakes "Sexy Back", but with different instruments and different lyrics, but damn did it sound the same. I wish i remembered what that soung was called. In my search i found this video which remidns me of Babel
http://www.youtube.com/watch?v=atNMZS7Rfuw
I know what you mean, covers and sampling can be good but the majority of the time its like an easy way to cash in on someone elses sucess. There was that song "S.O.S." by (insert pop artist) that, like "Right Round" was clearly just ripping off the 80's mega-hit "Tainted Love" (which ironicaly, is a cover from the 60s! lol). It was such a shitty sample too because the song was so horrible, ugh.
Meanwhile you have someone like M.I.A. that uses a very small sample of New Order's "Blue Monday" in one of her songs (can't remember which at the moment), and its used in such a way that most people don't even freakin hear it. Hell i didn't hear it the first few times I listened, but once i found out it was there i picked up on it and thought "wow, a song that uses another songs sample that DOESN'T rely heavily on it."
As far as straight up covers/tributes/interpretations go, I love em. Good mash-ups too, though there are some pretty crappy ones floating around.. yeesh. I love covers when they're done by an unexpected band, like how Deftones are known to cover Weezer songs. Now i've never been a fan of the Deftones, but i've heard plenty of their music (an ex of mine was obsessed) and never in a million years would I have imagined them covering Weezer's catchy tunes.. but they do! Surprises like that are fun because either they go for the straight up cover and they try to sound like the original, which is a fun surprise when the original song/artists sounds nothing like the cover artists... Or they go for the interpretational cover in their own style and makes the original song sound so different and new. A good example hear is Marilyn Mansons cover of "The KKK Took My Baby Away" on the Ramones tribute album "We're a Happy Family". Personally I don't like it much, because it takes away the catchiness and energy from the original, but it was a pretty ballsy thing to do when everyone else contributing to record did more of a straight cover.. Save for the Chili Peppers who had a nice rendition of "Havannah Affair". Check it out!
Now, mash-ups are very hit-or-miss.. most of the times you'll hear a crappy mash-up thats basically one songs lyrics over another songs melody, and maybe the chorus or a few lines of the original song mixed in. Thats LAME for the most part (one example is a 50cent vs NIN one floating around, "in da club" vs "closer". its not bad, pretty fun to listen to actually just sorta lazy). I don't mind this type of mash-up when done live, though, because its obviously simpler. There's an mp3 floating around of A Perfect Circle playing The Cure's "Love Song" but singing Ozzy's "Diary of a Madman" over the verses, but using Love Song's chorus (though slightly altered) and its really good.. i wish there was a studio version somewhere because the live versions quality isn't all that great.
A really good mash-up will fuse the two (or more!) songs together musically as well as lyrically. Two good examples of this, off the top of my head, a New Order & Depeche mode mash-up that mixes in "Bizarre love triangle" and "Strange Love" really REALLY well. I also have a sweet mash-up of the Bee Gee's "You should be dancing" and the Police's "Roxanne" which was cutely titled "Roxanne should be dancing" and that ones pretty fuckin epic.