Well if they all sound the same, it would be really boring. Also i know you like Black Sabbath, but that's old. Think of the modern rock, like ac/dc or Metallica (hard rock/metal). If you are thinking of going really far, try something different.
-Vocalist- has to be really good, he must practice every day at least 2 hours, His job is the easiest.
-Guitarist- Electric/rhythm guitar is good, acoustic guitar won't be used often in a rock song. This guitarist will play background music, this helps create an atmosphere and help cover the bassist so the song won't feel empty.
-Bassist- Needs to play really well, he should play main riffs or solos, also it has to sound louder than the guitarist, drummer to lead the song. This is the main piece.
-Drummer: Shouldn't play really fast (that's for metal) this member is perhaps the least important, but needed. The drummer should be a background type of thing, also it should never ever be the main sound in the songs. Try different styles, but his job is pretty much repetitive.
Try get the instrumental part to be the main focus of the band, if your singer isn't a god, don't try push it. Riffs and solos are what people are mainly interested in.