First, the atmosphere is something that is overlooked by a lot of bands. Say what you will about whether or not the band should still exist (I say not), but a group like Mayhem seems to put a lot of effort into the overall presentation. That does not necessarily mean that every band has to use blood and rotten meat in their shows. However, aesthetics play a much larger role in the creation of the proper atmosphere than a lot of bands seem to realize. In the case of Mayhem, it makes perfect sense to use candles and rotting flesh and fire and so on, with even the stench of death adding to the feeling. Many musicians get lazy or think that their mere presence is enough, but that isn't so. If a band plays music that is even remotely dark, then flashing neon green lights are probably not going to help the audience get into the right mental state. As well, the number of bands that get on stage in sweat pants, sports jerseys or any other normal and casual-looking clothing really show that they don't get it. The point is not that dressing a certain way makes you a true Metalhead or not, but if you are on stage then you should at least look the part. Lawyers don't show up to meetings with their weekend clothing and nor should Metal bands, especially those playing Black or Death Metal. That does not mean that you must have some sort of choreographed moves and synchronized headbanging, but at least replace the Garfield t-shirt with the pizza stains with something black. While the music is ultimately the most important thing, aesthetics and presentation cannot be ignored.
A poor set list can also ruin a show for many people. In most cases, a band's best work was accomplished in their earliest days. This is almost universal when it comes to Metal. Unfortunately, it seems to be quite common for bands to largely ignore their old material in a live setting. Naturally, one of the main reasons for going on tour is to promote a new album. However, that is best done by mixing a few new songs into a good mix of old classics. What a lot of bands appear to do is the total opposite. Or, even if they include some old songs, they still fail to properly cover their first album or so. Slayer is a prime example of this. Even by the early '90s, they were already disregarding Show No Mercy, with the exception of "Black Magic". One has to wonder if they remember how to play songs like "Metal Storm / Face the Slayer" or Tormentor". Even more modern bands are guilty of this. Watain only seems to acknowledge "On Horns Impaled" and "Devil's Blood", from their first two albums, despite the fact that the former is by no means the best song from Rabid Death's Curse. It only gets worse, as bands continue to release more and more albums; it becomes even more rare for the classic tunes to be played.
The next thing that is likely to ruin a show is a shoddy performance from the band. This could result from any number of things, some voluntary and others involuntary. Sometimes, bands just are not willing or capable of pulling off what they did in the studio in a live setting. In these cases, they should either rehearse more or stick to playing songs that don't require any studio trickery. A lot of times, musicians will just get too intoxicated to properly perform, ending up in a sloppy show for those in attendance. The primary culprit is frequently the vocalist, which can often ruin everything else. Over the years, from Megadeth and Metallica to Cannibal Corpse and Deicide, and so on, there have been plenty of times where the vocalists were in no condition to give their best and put on a rotten show with worn-out throats because they failed to take proper care of themselves. Or even more irritating is when they appear to forget lines or decide to change up the vocal patterns for whatever reason. This leads to the next point.
People show up at gigs in hopes of hearing their favourite songs performed as they know them from the albums. When the band alters the pace of a song, playing it faster or slower than the original, it ruins the atmosphere more times than not. For one, a lot of bands like to see the crowd headbanging or singing along and this becomes more difficult when a familiar song is turned into something hardly recognizable. If you want to take an already fast song and speed it up a little, that is no big deal. But it is a mistake to take a song that is supposed to be fast and intense and slowing it down so that idiots in the crowd have a better opportunity to mosh. The same goes for speeding up a slower song and robbing it of most of its impact. The time to edit the songs is in the rehearsal room or in the studio before they get recorded for an album. Once you're on stage, stick to the script and just play the songs as they are meant to be heard. Nevertheless, when a band is in top shape and putting forth full effort to do their best to play precisely and to offer a decent show, things still go to hell, more often than not. The main reason for this has nothing to do with the bands themselves, rather it has more to do with the audience.
People are stupid. Naturally, there are exceptions, but the masses are empty and worthless and would be better off utilized as landfill. A lot of people see concerts as a chance to go out and socialize, with no thought to what band is even playing. They are simply there to be seen, to whore around or just looking for an excuse to go out and get drunk. This is the worst offense, as these inebriated mouth-breathers ruin the atmosphere for everyone else, as they are typically engaged in some sort of idiot behaviour. They start fights, make a lot of obnoxious noise, stumble into people as they wander through the venue and generally annoy the hell out of everyone else. The worst is when these subhumans engage in moshing or crowd surfing, among other activities. There is no point in moshing in the first place. It is some sort of primitive and savage custom that is best left to the jungle-dwellers. Anyone that engages in this behaviour needs to be taken out into the alley and shot, point blank. Too many ridiculous people go to gigs for the sole purpose of thrashing around and slamming into each other and this really is not the place for it. Rarely do they ever demonstrate any connection, whatsoever, to the bands that are playing. It often turns out that these losers are hardcore kids that don't even listen to Metal. They just need some noise to add to their tribal mating dances, showing up to ruin the concert for those in attendance that actually paid to see the bands play. The same goes for crowd surfing, particularly those morons that choose to do so from the back and end up just climbing on others and kicking them in the head in their attempt to get up and going. The only amusing part of this is when the imbeciles get dropped on their heads by those that want no part in the whole thing.
That brings up another point. So many of the people are shows are obnoxious as hell and take give no consideration to those around them, in any way. Everyone is enclosed in a small space, trying to get a decent view of the band and pay attention to what they're doing, so anyone with intelligence would think that this would all be taken into account. But that doesn't keep the guy that is standing practically on top of your feet from headbanging wildly and giving you a taste of his shampoo... or lack thereof. It certainly doesn't prevent the other morons from stumbling into you, falling on you or the moshing idiots from slamming into you at the speed of a minor automobile collision. No one wants some drunken dumbass slamming into them every ten seconds, nor do guys in the crowd want to have to take their attention away from the band in order to focus on protecting their girlfriend from being smashed to pieces. In other words, there are still people that spend their money on tickets and go to shows because they want to experience the live performance by the band, rather than going there to be assaulted by strangers. The least these moshers could do is to avoid those that are clearly not participating in their idiocy. Otherwise, there is no need for the dirty looks or fights when someone gets tired of having a sweaty guy slamming into him and decides to shove him twenty feet in the other direction. It's typical of people, these days, to show no concern for those around them as they do whatever they want but to then get offended when someone infringes on their 'right' to negatively affect others. Who are the people that pay their money to attend a gig and to them ignore the band in favour of the aforementioned behaviour? Why even shell out the cash for a show if you intend on spending the whole time slamming into other sweaty men? You'd be better off going to some sort of gay bar and getting the hell away from those that want to enjoy the show, altogether.
Finally, even when the bands are on top of their game and the audience is relatively well-behaved, there are some things that ruin a show right from the start. It may be a personal preference, but there is no reason for so many of the biggest festivals needing to take place during the spring and summer. Nothing ruins the feel of a Black Metal band's set quite like baking in the sun, being drenched in your own sweat and being ready to pass out at any moment. Obviously, not too many people are going to want to arrange to play in the mountains during the middle of winter. Nonetheless, it seems that autumn would be the ideal time for such things. The weather is cool, but not too cold, and it certainly suits the atmosphere that a large number of Metal bands are going for better than the sweltering heat of the summer. Festivals, in general are irritating anyway and would probably still be incredibly lame even if they were moved to a less miserable time of the year. Everything is insanely overpriced, there are too many people and promoters try to cram way too much into the time and space that is available, sometimes. The tickets, themselves, often sell out before the full line-up is even posted so, by the time you realize that you're interested in going, it's either sold out or the only remaining tickets will cost you an arm and a leg.
Most Metal is quite misanthropic and solitary at heart, anyway. In all likelihood, the very best way to experience the music as it was meant to be is to make your own atmosphere. Turn off the lights and tv and computer and just light some candles and soak in nothing but the music, or maybe even read along with the lyrics and look through the booklet to appreciate the efforts that went into its initial presentation. Dealing with Jewish concert promoters and vendors that do all they can to rip you off, as well as the many annoying people that show up to gigs is only going to ruin everything. Even if the concert goes well, chances are that when you try speaking to one of your favourite musicians, you'll only find out that half of them are drug addicts and scumbags. One way or another, there are just too many chances for the whole thing to be ruined and it is all much more of a hassle than it is worth. Sometimes, a pleasant experience sneaks by, but those occasions are rare.