Currently, Stack Exchange sites use the CC BY-SA 2.5 license.
There's a lot of content (not the least of which is Wikipedia) under the newer 3.0 version of this license.
Content licensed under 2.5 is forward-compatible and can be used with the 3.0 license, but the reverse isn't true -- CC BY-SA 3.0 content can't be used here.
Is there a particular reason SE doesn't use the newer version of the license? Should it be changed?