I´m thinking about creating a bedroom-doom-metal-project

What makes good doom metal?

Well I suspect this answer will be different from every user on here. For me it's a multitude of different factors. Good vocals, great slow riffs, good drumming, creating a dark and doomy atmosphere, etc.

Probably the key to doom- and I guess, the thing that's hardest to measure- is songwriting that's solid enough to keep you interested despite the often very low tempos. That and guitar tone. 'Course this doesn't apply as much to the more rocking side of doom..

to me, it isn't just about "Slow riffs" and "Atmosphere." Those are defining elements of doom metal, to be sure, but to me, what makes GOOD doom metal is the same thing that makes good music of ANY genre. Namely, good, catchy, effective riffs, with a lot of feeling and intensity behind them. And hooks- the hooks cant be too big and/or blatant and have to fit with the rest of the song- but the point is, it has to be about effective songwriting. Just throwing together a bunch of slow, atomospheric riffs would make for very boring music- this is a trap a lot of would-be doom metal bands fall into (and even GOOD doom bands make this mistake a bit too often.)

"Songwriting that's solid enough to keep you interested." As he stated above- that is the key, and it isn't that easy to achieve- even with faster tempos but especially with down-tempo songs. There needs to be enough variation in riff, melody and tempo (even if subtle) to pull you in.

Well, if you're referring to my post with "slow riffs" and "atmophere" part I didn't mean that I like it for the sake of slow riffs, the slow riffs have to be good interesting and innovative themselves. And the atmosphere has to be creative and pervasive enough to keep me interested. What makes doom good for me wasn't as one sided as it may have seemed. Their are other factors i just included the basics.

Riffs.

That's it in a nutshell. You have to be very good at writing riffs that are catchy, interesting, and evocative. No other element is as important to doom as the riffs. I can point to dozens of doom metal bands that have everything going for them, but they don't have the riffs and ultimately end up boring as all hell.

Depends what s'pecies' of doom your making. If its stoner, I'd say that guitar tone and production are very important. I never want to hear really triggered drums, or trebly distorted guitars in this style of doom; I want organic, analogue/tape production with massive sub bass and awesome bluesy riffs. On the other hand, if your making more death doom or funeral doom, I'm not averse to hearing more death metal style drum production and guitar tone (ie. some of Crowbars material has pretty clicky kick drums and clear guitar tone, and I could hear this working with funeral doom).

Ultiamtely, riffs are the most important but a great riff will fall flat if its recorded badly with a wimpy tone.

Also, sparse vocals.

Thickness and intensity. You can be as atmospheric or slow or riff-a-rrific as you want, but if it's not meaty and if it's not intense, then there will be nothing of worth to be found.

Look, I've thought about the most appropriate answer for this question, because it is a great question. I will say ultimately that the defining factor for a good doom band, is nothing other than sincerity. I don't really want to spend time explaining what I mean by this, because I think you should ponder that word until it clicks. Trust me, things like black and death metal can be faked (obviously), but sincerity in doom does not go unnoticed.

It has to groove, not in the sense that Pantera grooves, but in the way that good Jazz or Funk grooves. Or Sabbath for that matter.

Riffs. Massive crushing riffs

I'd say it's a mixture of all that has been said so far... the guitar tone to create a good atmosphere, the riffs, the slow interesting drum passages and offcourse non-annoying vocals...

I'll echo what everyone has said. it all practically comes down to the riffs. After that everything else either elevates it to a greater release or a terrible one you don't bother with.

drumming doesn't have to be great but it'll help.

Vocals have to be pretty decent cause if they are ultimately terrible they can quickly turn anyone off to a band. like Serpentcult which I actually thought the music was really good but the horrible vocals completely ruined the band for me of ever enjoying them.

Crushing riffs with a powerful tone; some sorrowful melodic leads (like Paradise Lost or MDB ones, for example), solid drumming with a bit of variety and passionate vocals, being growls and/or cleans. If cleans, preferently a low baritone delivery.

Hopefully, some songs might feature some headbangeable moments and a bit faster riffs to counterpoint and accentuate the most crushing slow moments. No matter how crushing is your tone, if you'll be like DUN.... DUN.... DUN.... everyone will fall sleep.

Diagree totally about how you said there needs to be some counterpoint riffs. That's up to personal taste cause some bands like Toadliquor or Buried At Sea or Corrupted is all about DUN DUN DUN crushing tone and awesome [slow] riffs that are all a snail's pace and it's memorizing esp if you are a fan of actual doom. And I can name off more bands and quite a large number of friends and aquaintances that agree with me and eat up the slow and disgusting and some that actually don't like it if a band picks up the pace to even a rock pace.

I'm more of a death/doom fan than trad doom anyway, so I'm all in for a bit of variety. The drums might stay slow for the whole song, but for me a bit of changes in the riff department don't hurt at all. Still, I like funeral doom, but the bands I like the most are the ones who has plenty of different riffs, while keeping the doom mood right.

Like I said it's all personal opinion, I just don't like it when someone thinks their opinion is a blanket statement for everyone. I can jam on 40min long songs with literally one or two riffs. No variations of the riff. just that sweet riff smashing everything to pieces. If it's a good riff it's a good riff and should be milked right up to the point where it might be redundant. Sometimes that is 4 phrases of it sometimes it's 30mins of repetition.

I was waiting to see if anyone else would bring this up. For me this is one of the most important factors in separating the men from the boys when it comes to doomslangin'. Black Sabbath (along with most, if not all, 70s metal groups) understood how and WHEN to lay on the groove. If there aren't at least a few moments that you can shake your butt to then its probably falling in the "sleepy time doom" category for me.

Personally, the riffs have to be excellent (which seems to be achieved by many more doom bands than other genres), OR it has to have an absolutely disgusting, filthy atmosphere - the Wormphlegm demo is the best example of this to my ears.

That's it. Don't be boring. Be creative and memorable.

I was thinking more about headbanging, but that works too. Especially for attractive females.

I'd say, besides what has mostly been mentioned, is that you have to watch pacing.

I'm a fan of doom at about Black Sabbath and other traditional doom pacing, slow enough to be doom ridden, but not so slow that they drag out. If you drag them out, they have to be done in a way with strong hooks, along with not being drony like so many bad bands do nowadays.