[quote author="Alexandra" date="1323698877"]
Besides, wouldn't it be boring if it was "Mad Scientist, once and for all times"?
[/quote]
It will be, in the meaning that a mad scientist is nothing more than an old member that has being in the forum for a long time. But very active members will remain at the top of the ranking even with point expiration. While this is far and right for them, the result of the point expiration is that you are hitting the medium users. I don't have a solution for this, but it sounds to me this is not the right way to improve activity on the forum.
[quote author="Alexandra" date="1323698877"]
And, I do remember quite some people complaining about others gathering points for what they perceived as less valuable contributions.
[/quote]
That is why I believe that a thank-to feature, as implemented from vbulletin (for instance) will be great. Giving points to any contribution without any distinction is not very useful, giving quality based points it is. I don't have a solution for this either. Sounds like the score expiration is a too big problem compared to the aim.