Thanks for the input so far. Here are a few explicit questions that I have:
* Time: right now the data model assumes that the precision is given on the level "decade / year / month" etc., which means you can enter a date of birth like 1435 or May 1918. But is this sufficient? We cannot enter a value like 2nd-5th century AD (we could enter 1st millenium AD, which would be a loss of precision).
* Geo: the model assumes latitude, longitude and altitude, and defines altitude as over mean sea level (simplified). Is altitude at all useful? Should it be removed from Geolocation and be moved instead to a property called "height" or "altitude" which is dealt with outside of the geolocation?
* Units are currently planned to be defined on the property page (as it is done in SMW). So you say that the height is measured in Meter which corresponds to 3.28084 feet, etc. Wikidata would allow to defined linear translations within the wiki and can thus be done by the community. This makes everything a bit more complicated -- one could also imagine to define all dimensions and units in PHP and then have the properties reference the dimensions. Since there are only a few hundred units and dimensions, this could be viable.
(Non-linear transformations -- most notoriously temperature -- will get its own implementation anyway)
Opinions?