Diablo was monotonous, linear hack'n slash. If that's your thing it's cool, but it's not an RPG.
As a CRPG fan at the time, it still boggles my mind that such a title had any kind of success after the likes of Wasteland, Bard's Tale 1-3, the numerous SSI/D&D titles, or the Ultima series. Those days were glorious, and nearly all of those titles are still fantastic today.
Diablo did long-term damage to CRPGs IMO. It established that there was a larger market for completely linear titles than there was for titles where the player could actually decide what to do - or at the very least the order in which you do what there is to do. And as a result, it more or less vaporized any type of funding for non-linear CRPG experiences.
To be honest, I loved Diablo, but I never even viewed it as an RPG. It was just a mostly brainless, almost casual game to play after school. You could jump in and out for however long time you had and grind some XP and loot. My friends played it too, so at school we got to brag who had more Zodiacs etc.
I deliberately didn't include it because it's somewhat contentious whether it is an RPG or not and I had enough games to quote without risking triggering that discussion... but yes. Also very influential, even on Baldur's Gate itself, I'd say.
