Pros (going to list more just because):
1. Manly vocalists that can also shriek. Dickinson, Halford, Jon Oliva, King Diamond, Slauter Xstroyes' Jon Stewart, Blackie Lawless, etc. Doesn't have to be a precise style, but I like guys that can do gruff and also shatter windows. Or some of my favorite thrash guys like Keith Deen and Ulysses Siren's Manuel Lopez qualify too, being able to bark aggressively and angrily while still knowing how to carry a melody is a great quality.
2. Unannounced musical transitions. Sometimes you can get away with telegraphing your next riff; Exodus and most of the other early thrash bands did the thing where you play a rhythm two or four times, introduce a descending power chord over it a couple times with a slower drum track, and then play them both together at full speed for the verse. A common trope that gets boring after enough time, but Piranha for example wouldn't be the same without it. But in general, I don't want that shit, your rhythms probably aren't so amazing that they warrant an introduction, just get to the next part and don't waste my time. Early Megadeth was great at this, and on the lead-front as well; you don't have to repeat the under-solo-riff two or four times to let me know the solo is coming, just blare it out. Again, there's exceptions where a little announcement helps things, like when Araya briefly plays that hardcore-style bassline before Hanneman plays the best solo of his career in Praise of Death, but those are rare. Just get on with it.
3. Unique guitar tones. There are probably a few times where unique is bad, but for the most part, if it's different, that's a plus. Every great Destruction album sounds completely different guitar-wise. Adramelch Irae Melanox with its paper-thin midi-sounding tone, fucking beautiful. Shah's Beware, ugly-as-fuck buzzsaw, not even close to a favorite album of mine but the guitar tone boosts it a lot.
4. Unique anything really, I'm a filthy slut, internet piracy is my Grindr, I want to travel the world and discover myself with new and exotic riffs pumped up my ass, I'm too young to settle down. Even if you're some garbage black metal band, if you have something that isn't just an aesthetic gimmick, I'll feel like I didn't completely waste my time.
5. Minor 2nds just sound super cool and fucked up to me, especially for lead guitar. I think most examples of this, Destruction's Cracked Brain and Napalm Death's Harmony Corruption the first examples where I noticed it, just use some effect pedal to do it rather than trying to precisely replicate a solo one half-note off, but I don't care, it sounds sweet.
Cons:
1. Repetitive rhythms and meter. It can be fine in certain contexts, but if you have a habit of writing one-bar riffs that repeat themselves 16+ times, or a habit of writing tremolo-picked riffs where every riff is a perfectly even series of sequential notes, you probably should be playing lo-fi indie rock, not metal. 98% of black metal and 90% of death metal is useless because it fails to appreciate the value of even passable rhythmic change. Metronome drumming can obviously fuck off as well.
2. Vocal melodies that follow the main guitar melody, or vice versa. In small doses fine, but it can easily hurt my appreciation of a song (see: Iron Man, Battery, and every other post-reunion Iron Maiden bridge section). Just embellish it in some way, if you absolutely need some unison, then just syncopate the vocal rhythm slightly or something, a few words per verse could be sufficient. This is one area that Priest almost always gets right, even on the simple poppy radio anthems they understand the value of guitar-vocal play.
3. Sort of a continuation of #1 but I'm actually struggling, I'm pretty open-minded as long as you don't suck dick rhythmically: syncopated rhythms that are mostly just syncopated within a tiny frame but not the whole of the rhythm. I don't know the exact way to describe it, but a shitton of melodeath and metalcore qualifies. For an idea, first think of Metallica's syncopated, triplet-like rhythms, songs like Damage Inc, as well as the thrash bands that followed, like Testament, Believer, Stone, etc, you know, dut-dutalut dut-dutalut. The OG thrash bands would take those brief chugged phrases and fill them into a larger rhythm that does not fall directly on the beat, it's basically a riffing unit just like a single gallop could be a riffing unit. But then the melodeath shitheads came along in the early-mid 90s, stole that riffing unit and the kick-snare drumming backbone, and wrote riffs where each dut-dutalut lands squarely on the same beats for every bar.
4. Beat-counting-rhythms. One of Steve Harris' greatest sins, literally half of his songs go something like ONE and a TWO and a THREE and FOUR and a FIVE and SIX and. You don't get a pass just because you play odd time-signatures either. Dream Theater is rightfully ripped for that, but it's Unquestionable Presence's greatest sin as well; yeah you go 7/8 then 13/16 then 9/8 or whatever but all you did was play evenly-spaced 16th notes the entire time you fucking cheater. You fail the class, go back and study the first three Mekong Delta albums some more.
Metal isn't just one sound and different production fits different styles. That all being said, production is NEVER a make or break for me. I let the actual music make that decision. "It doesnt sound kvlt enough" is literally the definition of poserism
