On average I would say little for most cases.
Yes most of them I would say have SOME love of Metal, but even the more devout Metalheads and I tend to have heavily divergent tastes, for example:
Darth: Primarily Classic Metal and Prog.
Tron: Extreme Metal, w/heavy Black Influence
Peace Sells: Prog Metal with a side order of Prog.
Wakemeup: The More Raw and Brutal, the better.
ShredDoktor: Over produced tinny "Faerie Tale Victory ftw" Metal.
Brian: Tech, tech and more Tech.
Now my love of Metal tends to cover all the above pretty evenly, except I am not hugely impressed with a lot of Technical Metal, and find that a lot of Neo-Classical and Prog to be ego gratification wankery for elitists. But the point being, that even those who are predominately into Metal and I probably miss on taste more than we hit, as Tron tends to not really care for most Power Metal. Darth cannot deal with harsh vocals. Wakemeup and Peace Sells and I totally disagree on modern Thrash, ShredDok and I disagree on the usefulness of dirty production (which can be very useful... as can clean, when applied for the right reason) most of the stuff Brian likes is huge on technique, but IMO (and nothing more) very short on actual songwriting, and I find precision and technique with no emotion utterly boring. etc, etc, etc.
And each of them could similarly dissect my preferences and tastes.
For the less "Metal" oriented contacts I have, like King Crimson (who I have some pretty interesting and epic debates with about modern music vs the 70s), kls81, Lady Silver Rose, etc, we don't' have a huge amount in common, but meet in the middle on certain bands or styles, but not any great number of them. I have interests in a lot of the same musical "styles" as they, but have more surface knowledge of Rock and Folk, than my generally in depth, psycho obsessive knowledge of Metal. It's funny though, how in Folk and Classic rock and the like I tend to know more about obscure bands than the more popular and mainstream of the style.
But the reason I have them as contacts is not about "musical compatibility" but because they are people whom I regularly find give well thought out answers, and tend to be at least moderately "open minded" (well.. not sure about King Crimson or ShredDoktor... lol, jk bro!). They add in and their differences act as a compliment to my tastes and and accent and add flavor to my own musical quest. I find it laughable how many people come on here and only want sycophants and yes men to assuage their delicate egos by telling them how great their tastes are.
Really, they exist in all sorts, but I find a large number of the Queen and Beatles fans on here seem to be that way, and a ton of people who listen to the most mainstream commercial tripe (Paramore, Panic! at eh Disco, etc, are just looking for "validation", this need for affirmation of their self esteem.
Not that all of them are this way, just that it seems to be a vocal minority amongst them that simply cannot deal with anyone having a dissenting opinion. Heck, the most unhinged one I know is actually a fanatical Ritchie Blackmore fan, who verges on an E-stalker of the guy. You dare think anyone is a better guitarist, or mention that he is a control freak and wife beater and ZOMG!!! you get cussed roundly for a fool, and told you are a misinformed idiot.. because obviously this teenager at this stage in her life know far more about this musician than those of us who have been listening to him for 30 years, lol.
So I would say I do pick my contacts on admiration, but I do see tons of people here that do seem to pick solely based on those who share the same band tastes, or will just agree with everything they say (such as how many Queen fans on here literally only have other people with Queen related names and avatars as contacts.) Heck, one of the guys on here I actually respect most (and I don't remember if I have him as a contact) is Sugaree, because even though we have radically different preferences in music, I find his ability to distill an answer down to it's rawest essence to be utterly fascinating. (at least, when he isn't' just trying to yank someone's chain for taking themselves WAAAAAAY to seriously, like certain members,. past and present seem to)
Also I just noticed I never added you as a contact AtS.. mind if I rectify that? You are one of the "good guys" usually!
****Mind you these are rather broad generalizations, but not total stereotypes, And I certainly could be painted with my own stereotype by any of my contacts too!