Blut Aus Nord have released three fucking superb albums, the only album which can be described in that way is the relatively poor Mystical Beast.... I appreciate that the new one is an easy one not to like, but most people slating them haven't heard the first two I bet.
Deicide's Legion is certainly 7.5-8/10 material, the rest is overrated yes.
Most of Graveland's work is somewhat overrated, but almost all of it is at least decent. Thousand Swords, on the other hand, is hugely underrated by most, though perhaps understandably.
Possessed spawned a whole genre... of course they're going to sound dated now.
I can understand people finding Transilvanian Hunger boring, but Soulside Journey and UNDER A FUNERAL MOON?!
Burzum... not even going there, clichéd response but my GOD is it your loss.
Venom yes Arcturus (holy shit) yes Anthrax yes Testament yes.
Sabbath mostly yes, but a few songs fucking rule. Same with Metallica, Iron Maiden, Zeppelin and Exodus. Early Carcass is alright but yep; all their work's overrated. Ulver's BM stuff isn't that bad but it certainly isn't good, either.
Asphyx?! NO. The Rack remains awesomely powerful to this day. Obituary are a little overrated but Cause of Death still rules.
Hypocrisy were decent during their DM period but they were never anything to write home about. Some Amon Amarth is enjoyable though hardly stellar. Decapitated and Isis suck, yes. Death are overrated but have had a few alright releases.
Necrophagist aren't amazing but they're pretty good. Psycroptic, on the other hand, are terribly terribly overrated. Mithras were overrated by Terrorizer and a few people around these parts, but I like them.
Solitude Aeternus are pretty good and I don't see them mentioned much. Early Cathedral rules, but everything since then does not (and to cap it off they suck live). King Diamond... yep. Dissection yep.
The Chasm? Fuck off, Thraxz. It's even more impossible to overrate The Chasm than it is to overrate Hell Awaits, and that's saying plenty. Some of Nile's work is overrated, agreed, some of it is deserving of praise though.
Now for a few of my own (I'm thinking of "extreme metallers" here):
Watain, Averse Sefira (though the new one's really good), Angantyr, Ancient Rites, post-BS Therion, Cryptopsy, Evoken.