Maybe they get airplay as a major influence, but their riffs aren't dark, they aren't all that heavy(I mean, they're distorted, don't get me wrong, but to me, in my mind, not heavy enough). To me, ACDC is the quintessential Hard Rock band. A band like that is where I draw the line between 70's Metal and 70's Hard Rock. And to me, there IS a difference. They even say themselves, they're just a Rock 'n Roll band. Meanwhile Priest have always called themselves a Heavy Metal band. Take that for what you will.
For me, the litmus test is Priest and Sabbath and then later the "New Wave of British Heavy Metal" all who are heavier, darker etc than anything I've heard from ACDC.
Sabbath was the heaviest band of the 70's. They, partly because of Iommi's hand injury, were heavy. They were a band that heavily used the tritone, had one of the eeriest vocalists in Ozzy, had dark lyrical content that became a staple of Metal... they essentially started Metal as we define it today:
Dark, Heavy, Evil, Menacing... and later, before anyone else, Fast.
I've never heard an ACDC song and said: "wow, that's so heavy and dark". Sure, Highway to Hell has somewhat dark lyrical content, but it's such a jovial song compared to what Priest and Sabbath were doing, I can't see how you can say they're the same. Their tunes form the 70's sounded tinny compared to what Sabbath(especially Sabbath) and Priest were doing. Thunderstruck? Yeah, that song is heavy at times and that song without a doubt had major influence on Metal, but again, it doesn't check enough boxes imo, ie lyrical content, or that evil sound I'm talking about.
I mean, along with the sound, it's the subject matter. The notes you use have to be dark, as well as heavy, but the lyrical content, particularly in the 70's when Metal was still being defined. ACDC's lyrics don't talk about dark subject matter the way Priest and Sabbath did. They didn't use the same level of distortion that both of those bands did. Not back then.
Here's the criteria I'm looking at here:
1A: Heavy Distortion: Sabbath and Priest both had good distortion, even early in their careers. Later in their careers ACDC couldn't touch them in that regard.
1B: Evily sort of sounding riffs(to quote Iommi): Listen to Children of the Grave(Sabbath) or The Ripper(Priest). There are tons of songs by both of these bands that have that evil sound I'm talking about.
2A: Dark Subject Matter(Lyrics, art, etc): Practically every song by both bands has this, outside of a handful of them.
2B: A gritty, distorted or menacing vocalist(This is where I think ACDC are a huge influence on Metal, I'll give them this)
Same goes for 70's Zeppelin, Heap, Purple(although some songs I'd consider pretty close to Metal)Thin Lizzy, even Rush for the most part(Anthem has a seriously HEAVY intro though)... along with those bands and ACDC they had a HUGE impact on Heavy Metal, but were they actually Metal? I'd consider most, if not all their works to be Hard/Heavy Rock, but most of them, particularly ACDC are missing a few key elements to qualify them.
Again though, feel free to disagree. It's just my honest opinion
Guns N' Roses 
