Led Zeppelin - the band deserves a lot of credit but also deserves a lot of criticism. I won't go into anything lengthy here as my point in this post (on topic) is that Robert Plant sucks. ok, maybe doesn't "suck", but he's NOT a good lead singer. By far, Plant was the weakest link in the band. Bonham's drumming is legendary. Page's licks are legendary. JPJ was damn good at keeping up the bottom end and was really the only person that didn't look or sound sloppy on stage in the 70's. But Plant? The guy shrills and shrieks far too much. It's painful to listen to at times. He sounds great on some songs and I'd rather jam ice picks in my ears while listening to others.
I know he's gotten plenty of guff from fans over the years for not wanting to do a LZ reunion. For the fans, it may have been a good thing. But he always refused (other than the couple of one-time-only appearances they did). Aside from him seeming like an asshole, I'm fairly certain that the biggest reason he didn't want to do it was because he can't. His voice wasn't good in the 70's. in the 80's and 90's and beyond, he's been fine with his solo stuff because he doesn't sing in the style of LZ anymore (again, because he can't).
I own every LZ album (well, the original stuff, not the 55 re-releases, remasters, etc that keep coming out). When I listen to any of it, it's usually for the music and LASTLY for the vocals.
Sidenote: This is also why I can't stand that craptastic band Greta Van Wannabe. JFC, that kid makes Plant sound like Pavarotti. The music in that band is ok, but sounds 100% like copycat music, however, that kid on vocals is one of the worst things I've ever had the displeasure of listening to. Watching him makes it even worse.
The TL-DR version? Zeppelin was good, Robert Plant is awful.