There are certain markers that one looks for in order to judge a singer as "great." Stage presence, musical ability and good looks are definitely important. But there are a ton of people who have these skills. So, what sets apart a person like Whitney Houston, for instance?
Whitney Houston was arguably the greatest female pop singer of all time. She had everything - the look, the charisma, and the sound. It wasn't just a pretty voice, it had depth and character. She could give you chills with her high notes and at the same time, touch your soul in smooth, lower passages. What is most interesting is how she handled diction. She NEVER overdrove the words or fronted her vowels - a rarity in pop singing. She approaches all her phrases with huge vowel sounds that are placed a little further back in the mouth to accommodate the stretch of her soft palate and constrictors in her neck. She also makes the vowel more important than the consonants - she delays the ending consonants ever so slightly on longer phrases in order to really allow the vowels to spin and create that chilling affect that we all love. Finally, she also uses a wonderful mix of resonators to add drama and pathos in all the right places. Mixing head and nasal tones to introduce a theme, then adding deep chest tones as things heat up.
Below is a clip of Whitney singing her first big hit, "All at Once." Note how she never contorts her face to make the words or emotion. It's all very relaxed-looking and just seems to flow out of her in an effortless fashion. She is totally in control here, and it shows.