I'm saying that Opeth ISNT a doom band, ready my post again matey and restructure what you have said here (because I don't disagree with you, I only disagree with how you have put it across).
The closest that Opeth have come to doom in my opinion is that Mike did vocals in BMD, which was a doom record (people will argue on this one too).
If you're into doom as much as you say you are, then you should know that many people are quick to label something doom just because they hear sadness or see lyrics about melancholy! This DOESNT catergorise what doom is. Then also take into consideration that "doom" (eg Cathedral) and "doom metal" (eg My Dying Bride) are also two different things as well (although now I head into the sub-genres debarcle which has already been discussed).
From the thread on doom-metal.com: ""Opeth mix Rock from the 70s with Death Metal. Their music is way to fast and contains to much triggering to be doom. Also, their lyrics doesn't concentrate on sadness, apocalypse or whatever."
Doom is a completely different genre of music as to what Opeth are doing. While both genres take influences from the 70's, the influences are massively different ones. Opeth take progressive rock, while doom takes Black Sabbath (you should know this already!). There are no similarities between the two, they are obviously different. Not only that, but the lyrical and emotional focus of doom goes much further than Opeth does, it's a far darker and fatalistic form of melancholy. Doom is an extremely dark romantic form of metal.
For those new to metal (I have been into metal since 1986), I suggest you read the following for more info overall. These are good articles. Reading band interviews is also a good thing too, as the band will say themselves what they believe. There's much more available to ready, but these quick two are ones I could think of:
What is Doom Metal?
http://www.doom-metal.com/articles/article003.html
Quick notes on Metal Genres:
http://www.bnrmetal.com/pages/genres.htm
