3. Performer that has appropriate dynamics.
That really should be number 1. If you've got a compelling performance of interesting material, you can get away with a whole lot. Heck, if you manage to get a truly singular and inspired performance of really good material, people will just be glad somebody captured it at all. Robert Johnson made about 20 recordings and they all sound horrible, and everybody just wishes there was more. Bust out your camera phone and shoot it. The world will thank you. (Though actually these phones nowadays aren't as bad as you might think. They'll surprise you sometimes.)
OTOH, if it ain't worth recording then again it doesn't much matter how you do it. If it sucks, it sucks, and fancy studio tricks aren't going to help.
I would argue that single mic or stereo pair really is the right way to record a lot of acoustic singer/songwriter material. That's what it fucking sounds like! Record it. If you don't want it to sound like that, why are you even doing it? Do something else. Do you really need/want it to sound all polished and hyper-real? You real need to compress and eq the crap out of each instrument? You'll need to record them separately. Period. I'm not actually saying that polished/hyper-real is wrong. What I'm saying is you can't do it well by recoding both at the same time. But if you're happy with what you're hearing in the room, then record it. You're about done.
Yes, that means a good performance first. Then a decent room. Then you need to move things around until you like what you hear. I will always start with mic somewhere off center from every dimension in the room and the performer a couple feet back. Then listen and tweak from there. A good way to do it might be to just move around the room until you hear the balance and tone that you want and then put the mic there. Course, that's a lot easier when the performer is not also the tracking engineer.
Get a friend or your kid to move the mic around while you listen on headphones or whatever.
Sometimes in small rooms it's best to put the mic right at one of the boundaries. If you've got low ceilings, putting the mic on the floor or the ceiling can alleviate a lot of the ugly quick reflections, almost making the space infinitely large in that direction. You can do this with walls too. We usually want to stay out of corners, but rules are meant to be broken, and if it gets you closer to what you want to hear...
You don't get any magic answers, though. If you have reasonable expectations, but know what you want, and you are willing to put in the time and effort to find it, you'll come up with something that nobody will complain about. If you just stick the mic where somebody told you to and then wonder why it doesn't sound exactly the way you thought it would...???
Edit to add - I try not to point the mic right at any flat surface - especially like walls. It's not something you hear very often, and I'm sure in the grand scheme of things, it's a minor concern, and if you really wanted to do it right, you'd do the old mirror trick, but usually just angling the thing a little on every axis works well enough. There's a lot of math (and possibly some neurosis) involved in why I do this, but I think it helps, or at least is a safe place to start.
