Craftsmanship
Craftsmanship is an important factor in determining the value of a piece of furniture. Sometimes, when the craftsmanship is superb, a piece made out of elm wood can be more valuable and collectable than a piece made out of hong-mu (blackwood), all other things being equal.

The level of craftsmanship is assessed by looking at the proportion of the details, the accuracy of the joints, and the piece's fluidity, complexity (or simplicity) and dynamism.

Rarity
This is actually a supply-and-demand issue - if a certain style is not easily available in the market then pieces in that style are considered collectable, and their value in the market goes up.

For example, when the trend in the market is for classical Ming-style furniture but not very many pieces are available, then the price and value of pieces will increase. Similarly, pieces with special features or unusual functions tend to be more valuable. For instance, hunting chairs, which were rare in the old days, could easily be ruined simply due to the conditions in which they were used, so not many of them have survived. They are therefore considered highly collectable, and their value has increased over time.