Mastodon and Lamb of God opened for Metallica last summer in CPH when Metallica played 5 sold out concerts - I was not too impressed but I've moved a bit closer to the more extreme side of metal since then, so maybe now...? 
I have listened to a few tracks by CoF - he sounds like a twat to be honest... I saw Hammerfall at a Monsters of Rock in DK a few years back where they played as the first band of four. The three otheres were Rammstein, Accept (!) and Slayer. Hammerfall were crap to say the least!
I guess I'll have to learn something new then because I like the music - so time to be openminded and willing for new impressions! 
I'm listening to Amon Amath right now - they're really good! Perhaps a better place to start for me... 
All depends on what your ear attunes to really, ie what you like. If you prefer the vocals on AA to black metally stuff, then there's other bands that i could direct you towards. Certainly see if you can get hold of Clandestine and Wolverine Blues by Entombed groovey swedish death metal. If you want stuff a bit more technical with still a bit of groove, but a more modern production (although clandestines chainsaw sounding guitars are a beautiful thing to me, even after all these years
) you could try Daath, both their albums are very good in my book.
Shit there's tons of death. Hypocrisy, Dismember, Malevolent Creation, Deicide (first 3 albums and stench of redemption only though), Morbid Angel are some off the very top of my head that i would recommend, all of them can be heavy as fuck, but all can groove and thrash with the best of them at times.
Like i always say it's personal choice, you've just got to keep trying and find the stuff you like. Thank fuck you've got the net though, back in the late 80's and early 90's a lot of it was pick one and hope!
Dani filth is a twat, that being said, their first album is still excellent (and by far their best) as he's not being a squeaky gimp. Hammerfall are crapola! I agree! (but there's a fucking ton of people who would disagree with us
)
Oh, one album i definitely recommend to anyone looking for a way into extreme metal is Heartwork by Carcass. I personally don't think it's their best, but it's accessable, groovey, still got a bit of fair bit of grind and excellent vocals from Jeff Walker, as well as some great solo's from Bill Steer and Michael Amott.
Speaking of Amott, you might like Arch Enemy, who are fucking awsome live to be honest and Angela Gossow has a killer voice. They can go a bit solo heavy at times, but it's fast, it's heavy and in places very groovy.
