I am a guitarist, but 98% of the time, the singer Is what breaks the band (as No-Mercy says) but not necessarily makes it. Yes there are exceptions. Yngwie has had singer after singer, but has made most work fine.
Metal Church was an example I like as well. I actually like the Wayne era, but it pales in comparison to the Howe era.
I do have to disagree with the Skynyrd though. Though I love Blackfoot, I always just considered Skynyrd "ok." I don't think Johnny Van Zant is as good of a singer as Ronnie was, but with Medlocke (former Blackfoot vocalist/guitarist) now on guitar, LS is more heavy and I like their new song as much as any "classic" they have.
Bryant