Ever sit in an audience watching a singer perform and hear the comments from the audience members around you? I find it fascinating to hear what the average person finds impressive. I've heard horrible singers who are tremendously loud, sounding like they're grinding their vocal cords into hamburger meat while the audience, including older folks, are clapping like there's no tomorrow. Well, then and there, I made a mental note: regular people are impressed by loudness. Doesn't have to be good. Really loud will suffice. After that experience, I began focusing on other vocal skills which might lead the average non-musically-trained audience to exclaim S/he's good!
Turns out there are (at least) five factors which make a positive impression on listeners: pitch (meaning singing in tune), high notes, loudness, being ringy and being able to sing with vibrato.
I thought I'd take each of these skills one-by-one and dissect them a bit. Singing in tune is an incredibly difficult feat and it's quite amazing that we can do it at all, since there is no visual or tactile way for singers to produce pitch. It's all guesswork. Every semi-tone (aka half-step) can be separated into 100 cents. Some people can hear variations as little as 5 cents sharp or flat from the true pitch, but that's rare. Most listeners recognize out-of-tuneness around +/- 10 cents. Doesn't give singers a lot of wiggle room.
Singing in a pitch range higher than normal speech range seems to be quite impressive to the average Joe/Jill. We all can make high pitch sounds, but as adults don't do it often (neighbors complain). So when we hear an adult singing a high note, especially IF it doesn't sound strangled AND is held out, listeners eyes will pop out in amazement.
Loudness gets people's attention. Again, it's not loud is difficult, it's just not done in polite society with the possible exception of sporting events. If you combine high and loud and in tune, you've not only got people's attention, you can keep it.
Amplification of ring harmonics is related to loudness in that the piercing, metallic sound associated with ring carries sound, thereby making our volume sound even louder. Ring can even make medium loudness sound loud, so I call it cheap volume, rather like turning up the treble on your amplifier on your stereo (do we even have these anymore?)
There are two basic ways singers can hold a note: with straight tone or with vibrato. If you're a classical singer, you'll be singing almost exclusively with vibrato but in most other styles, vibrato is a volitional ornament used on held notes and end notes. Though there is an infinite assortment of vibrato types to be heard, the average listener tends to like whichever vibrato type comes their way.
Winning over an audience is a bit like a video game: you try to rack up points any way you can. Keeping in mind these 5 technical goals will help you plan your strategy to win.