advice How to rate the rareness of domains

What factors would you use to measure the rareness of a domain?

Currently considering a combination of the following:

• character count (the fewer the better)
• TLD desirability (.com > .io)
• age (how long ago was the domain first registered)
• real English words

What else should be considered? How would you rank these in order of importance?

My site Zephie tries to rank how rare a domain is and assigns the domain "zephs" - rareness points. Would like to make the points reflect how collectors think about how rare a domain really is.