"It's a bit difficult to pin down anyway as I've always found "grunge" to be ill-defined, as it somehow encompassed all of Nirvana, Pearl Jam, the Melvins, Mudhoney, and Soundgarden."
Not to forget: A grainy, low-fi production sound. 
We wouldn't want to sound like we recorded this in a state of the art studio now, would we?
And dropped-D tuning of course. What horror if the E string had the correct tension! 
Okay, I know you're taking the piss, but I can't help it
... Certainly there was a fair amount of drop-D going on at the time, but most of the big Nirvana singles were standard tuning.
"Jeremy" and "Alive" are standard tuning.
And few of the major-label "grunge" releases ("Nevermind", "Superunknown", "Ten" et al) sound at all low-fi to me.
Earlier records released on Sub Pop et al, sure - but that's because they
were low budget indie records, and comparatively few people actually heard those.
Once they actually had budgets to work with, probably the one major outlier is "In Utero" where Nirvana hired Albini to deliberately make their second major-label record sound as un-radio-friendly as they could.
I think Wikipedia hit on a common thread though:
"Lyrics are typically angst-filled and introspective, often addressing themes such as social alienation, self-doubt, abuse, neglect, betrayal, social and emotional isolation, psychological trauma and a desire for freedom." Take away the gadgets and NiN could've been Grunge
