I think that when someone has studied and played music, their ear is often a bit more trained to actively listen and identify things. That's not to say they're better or like music better, but they can often identify things in songs that the average listener isn't in tune to or noticing--or at least if they're noticing can't pinpoint what they are noticing.
I have played guitar since 1995, and spent a long time working in a music store. I played in a band for years, and two of the guys in the band have gone on to play music professionally (my drummer is now the drummer for Brazilian band CSS, my singer and close friend is in Rock of Ages at the Venetian in Vegas).
I think it is pretty obvious that the singer is the most identifiable person in the band for most people. Why? They tend to be "out front," but words are the easiest thing to identify with and mimic when listening to music. The easiest thing to latch onto is to start singing with the song. But are most people also picking up their instrument and learning the guitar parts like I often do? No. Do most people listen to, say, Foo Fighters' "Times Like These" and ask themselves, "Hey, that riff doesn't sound like 4/4 time. What is that?" and then proceed to count it and realize that it's in 7/4 time?
Probably not. However, most of you have probably subconsciously noticed things about songs that you just can't put your finger on. With singers, language, and melody, it's easier to put your finger on things.
I'll give you an example. If someone told you that David Lee Roth and Van Halen were back together and did an album and said, "There's one song on there that sounds like it came right off an old record," and played you "She's the Woman," your initial reaction (And mine) would be, "Hell yes. This
does sound like classic VH." Here's the song:
[size=78%]http://www.youtube.com/watch?v=Tj3ul98CnRg[/size]But here's where some people may notice something is a bit different and not know what it is. First, listen to the backup vocals during the chorus ("She's the woman / She's the woman"). David Lee Roth is doing his own backup vocals (about 50 seconds into the video). But it sounds fine--he's the singer!
But wait--this song actually predates the first VH album. It was part of the demo Gene Simmons did for VH. So now listen to the chorus with the original lineup:
[/size][size=78%]http://www.youtube.com/watch?v=og7hRK8kBdg[/size] (about 40 seconds in). Most people probably didn't even know that was missing--but isn't
that the classic VH sound? Michael Anthony was a HUGE part of the vocals in the band, yet the average listener, if it was never pointed out, might not ever realize it.
Now, I could take this further and point out that there is a much more rounded attack to the bass (check out the intro in both versions) when Michael Anthony is playing versus Wolfgang Van Halen, and that's because Wolfie plays with a pick and Michael plays with his fingers.
But all of these subtleties make a big difference in the final product. I notice that Eric is so laid back--so Bonham--that he's almost out of time. He hits each drum at the last possible second before the song falls apart. This was a huge thing to overcome for Ray Luzier when he stepped in, because he is on top of the beat, like Matt Cameron--but STP songs don't work that way. When Duff played STP's stuff, he never had the bass lines right. Robert's lines color all of their songs in beautiful ways. If that were to leave, people may not be able to point out that that is what is different, but it would be different. Same goes for Dean and his unique chording and solo style.
It's just that a difference in a singer is much easier to identify as different. But the rest of the band makes a huge difference too.