a many-fold answer.....Rarity, unique beads always draw me to them....quality, a top quality bead of a common bead may stir my passion........beauty, of course with all Objet d'art, beauty is a draw for me......Size, an unusual size variation of a particular bead type may be a deciding factor for me....value, a beads value may dictate my collecting it......market trends, as we collectors mature in our tastes, we sometimes find that we are drawn to different bead types at any given time, some may be because of popularity of certain beads, other times because of value trends in the markets, I try not to buy according to resell value but I am a realist and I know someday, maybe, I can sell my beads and not at a loss.....budget, and the #1 criteria for collecting beads would be the budget for them.....I would own all the Man-in-the-Moon beads if I could afford them...etc, etc, etc....we all live within our budgets and must collect accordingly!