A couple of thoughts from my own experience. First the too loud, too quiet thing. Choice of mic is a factor here, as well as how it is set up. I went from a condenser to a dynamic, because of outside noise. The condenser was on a stand, but I don’t have one at the moment, but it’s not to big a problem to hand hold a dynamic mic. If your mic is on a stand, set up a pop filter at the correct distance , so that when your nose is touching it you have the volume and clarity you need. You can use some compression to help keep things even. This is the only time I use compression.
Energy. Now this will be dependent on what you are singing about, and what genre it is. Your accompanying music will also effect this, so you will need to keep that in mind when song writing.
Sincerity. This all comes down to practice. The better you know your lyrics and the timing, the more sincere it will come across to the audience. Lyrical content will also be important. Don’t be too verbose. It makes it a PITA to get the timing right, all the words in and breathe at the same time (unless you are Snow or Hugo Weaving).