My system uses the date format dd-MMM-yyyy. Most of the dates on SE/SO sites are formatted as MMM-dd-'yy.
Having only 'yy makes it especially hard to search for years. yyyy would make that a lot easier.
It would be really cool if this became user-configurable; I personally like the ISO 8601 format very much.
No matter what, the default should change; only about 33% of SE users are from the USA, and only about 20% of SO users are. So it makes sense to go for a form of DMY, as that is what most of the world uses.