Mild fans are usually people who are a bit obscured on the thin validity of the word. Hardcore fans (of anything really) can become fundamentalists about a foundation quickly put together after smoking a tiny shake joint and thinking about how cool they need to be in order to become a real person with real things to say.
I think I'm gunna start a new business. Candy products, suckers mostly, plaid colored...call em grunge suckers.
On a personal note, I love Nirvana. I just hate most their fans.
For a good example of Grunge Fans, go on myspace and search for any band with the word grunge in their genre description. Although this can be used by a band which is actually cool and just having fun with the genre function of the website, you're best bet is that you'll land on a bunch of confused long haired teens with self-ripped jeans.
For another example, youtube anything by Nirvana and read the comment section for a bit.