Vihris-gari, I agree with a lot of what you’re saying but still, I don’t believe an ideal criteria is obtainable - I’m mainly just pointing out the difficulties in doing so.
One can come up with good reasons to say why or why any band isn’t metal if you judge them band by band (but of course, that would open up controversy, if certain points are contradicting). But, if you’re adhering to a criteria for all bands - I just don’t believe this works.
With the Deathstars, I was mainly pointing out here, that bands can come from a metal background and produce music that isn’t metal, but could possibly (?) retain metal influences that are debatable in the band’s sound - the band may believe they do incorporate metal into their music, and often when a band comes from a metal background it is easier to persuade those into believing that the metal elements are still there in their music. On their myspace they are listed as "industrial / gothic / metal." However, they aren't metal.
A singer involved in some fairly-known industrial and goth bands recently said in a chat the Deathstars weren’t industrial, they were metal. Here I’m just pointing out that when a metalhead uses the term industrial for example (or gothic or whatever else) it often isn’t completely accurate or to the point so that diehard industrial fans (for example) would ever accept this usage - just like when a person who doesn’t listen to metal says Limp Bizkit or Korn is metal, metalheads could never accept this because it isn’t true, and certainly not to them.
To come up with a proper criteria, (or as you mentioned, perhaps one that cancels out other genres in the process) the people making it should be well-knowledgeable on all genres, not just metal.
A style of music, originating from rock, which has the following characteristics:
(1) the use of one or more highly distorted electric guitars
(2) a reliance on emphatic rhythms and drum beats to achieve a "heavy" sound
(3) a dramatic or aggressive vocal style which emphasizes the vocalist's tone of voice over the lyrical content
(4) a focus on instrumental power which reduces the role of the vocalist's personality relative to many forms of rock music
I believe this is better but it still opens itself up to many "nu-metal" or heavier rock bands. And also, there are many aggrotech/terror ebm groups who already qualify for the last 3 points, and have implicated that they may choose to incoporate guitars into their music (seeing as some of these electronic-based bands are influenced by metal also). I'm sure there are some who have already done this, but I'm just not fully aware of them. Perhaps added to the list, should be a requirement that bands' primary influence be metal - but here again, this is highly debatable. Otherwise you could have like I mentioned bands from a completely different and unrelated style of music making the list should they add guitars. And I don't believe you can make it a requirement that instruments be 'real' as opposed to electronic, because many metal bands use drum machines. And what if a band was to have come from a non-metal genre, and unintentionally evolve their sound into a metal one, they wouldn't be originating from a rock/metal form and the core of their music would be debatable - I'm sure there are examples.
Also, I'd like to point out that "nu-metal" isn't a real genre. It essentially was an ignorant term used by many naive mainstream critics and whomever, to call what they believed to be modern metal via bands like Slipknot, Korn, Mudvayne. Most of these bands are (arguably) a more modern form of hard/heavier rock - I'm sure we can all agree on the ambiguity of how general, the term rock is. Many "nu-metal" bands don't incorporate rap/hip-hop into their music (and even Slipknot make little use of rap, they actually have more metal influence than rap). What defines a fusion genre? Would bands who mix black metal with electronics not be considered metal?