beyond the most obvious like having healthy or straight teeth. but there are smiles that just draw you in and just do something to you, maybe something about the person inside but what makes a really wonderful and contagious smile?
When it seems natural and conveys happiness that makes a great smile. And the rest of the face cannot be forgotten either. Your eyes and cheeks are just as important as your teeth in this case. A great smile has it all, seems natural, and is just warm and inviting. It makes you happy to see them happy.
A smile of innocence and pleasantness is generally what catches my attention. Bimbo smiles aren't attractive at all, but a sweet smile and gentle eyes is just too much to bear without bursting a smile.
What makes it contagious is what you radiate from the inside. You have to look at the person directly, use your smile to tease a little and coax a mirrored smile from the other person. I've gotten a smile out of people who apparently can't stand to be around me simply because I'm positive and direct, always greeting them with a smile and even smiling at parts of the conversation that seem either humorous or unbelievable.
Your teeth don't have to be straight or perfect -- look at Isabella Rossellini ( link )...
My secret? Honest eyes, a clear conscience and a positive attitude.
Someone's smile can say a lot about them: a cheeky grin can mean they have a sense of humor or are fairly mischievous, or a gentle smile can mean they're friendly and kind. A great smile is when someone expresses exactly how they feel and each time they smile, each expression means a different smile. A great smile is the ability to make someone else smile when they see your smile. The corners of the lips, the widening of their mouth when they have a vivacious personality/laugh etc.