What should clan ranks be based on?
Probably all of them together but most of all time spent in clan
If you're a map making clan - experience in map making.
Skill+Time in clan=Higher rank
Just because your good doesn't mean you deserve you should automatically go up to co-leader. You have to prove your dedicated to the clan.
Amount of hot sweaty sex per week. That's why Clan (U) is well known today.

QUOTE
If you're a map making clan - experience in map making.
What category is that best under?
I think ranks are rather pointless other than leader and co-leader.
But if you must:
Seniority (not the same thing as time spent in clan) and active participation such as making maps.
I would say-
Leadership Ability
Skill
Seniority
I wouldn't let someone become a shaman in my clan if he was awesome at the game but had no leadership ability.
I would say the way they treat the members in the clan, their skill, if they have a chance of good leadership, and being active.
By the way, when I said content made I was meaning you much stuff you have made. (Amount)