If marking "out of date" or "cleanup" etc, just a few words of why you think so might be helpful, otherwise marking work may do more offence than good.

I didn't quote a number for new signees. I quoted a number for existing mappers, from a mail on talk@openstreetmap.org.
So the calculation about the 12,500 mappers.
If I was to be concerned about the average 38 mappers per day quoted on http://neis-one.org/2010/11/23/short-update/ then my numerator would be the two or 3 hundred thousand persons signed up to OSM in total.

And the ODbL - actually I have read it.
It says one OR the other. It doesn't say who decides, and then it is decided by courts and lawyers. If the licence was fully written out, it would say at whose discretion you got a planet dump or got a script.

Although the tags in OSM were intended to be the same the world over, they are not in actual use the same. So any person sitting in one country decreeing how anything is mapped in another is going to run into complaints. People have used tags in slightly different ways all around the world. You cannot just change somethings and expect all of the changes to reflect the situation on the ground. Some will be correct and some will be wildly inaccurate.