It is the confusion and indefinability which makes it so special. No one will ever completely agree on what characteristics make someone attractive, but some things are pretty much given. Intelligence is a must. Intelligence is one of those things that can make or break it when it comes to being attractive. A sense of humor also is of the utmost importance.
Life is much too difficult to be serious all the time.
A person who can make you laugh is undeniably attractive. There are so many other qualities that make people attractive and what I find interesting, another person might think deadly. That difference is what makes attraction so wonderful and so unique. The fact that I really like a quality in a guy that another girl may not, that propensity to sing from the Beatles' repertoire, for example, makes me who I am. It makes each relationship I have different from the next because perhaps I found a little part of me in another person.