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.