Playscale Poser
There are two main types of "doll" collectors: Investors and "deboxers." Investors buy with an eye toward holding onto a figure in a well-preserved box for later resale, speculating that its value will increase over time. They would be more versed on what contributes to their perceived value, but I suspect limited runs and/or big name sculptors and designers play a role in such. Deboxers buy "dolls" as contemporary art in some fashion or another. They are less concerned about future monetary values as they are about the figure's aesthetic value in some type of display. They are modern, attainable, posable sculptures for the bourgeois, whether they stand alone, display clothing collections or are elements in a diorama. Unlike static sculptures, posability allows one to change the attitude of a display, while redressability allows one to change the theme altogether.

cltd
People collect dolls for many different reasons. Some for investment as mentioned above, but most doll collector's I know are like myself and collect just because they love dolls. I started collecting when I was young and was told I was too old for dolls. Some of my friends started collecting when they longed to have a long lost childhood doll back. What makes a doll rare is limited numbers originally made, made from a very fragile material that was easily broken or dolls that were not big sellers when they were originally sold. What makes a doll special is varied, depending on the collector. A well played with and very much loved, but common, play doll will be priceless to the original owner. Dolls like the original 1930's Shirley Temple dolls, while not rare, are special because of the little girl the represent. Value is based on rarity, desirabliity, originality and condition.