Fashion has gone through some pretty dramatic shifts over the past decade. I could go through an itemized list of changes but there's an easier approach: go to your social network of choice and compare pictures of younger friends or relatives. I cringe when I look at 2005 fashion, it's not remotely comparable to 2015 fashion.
There was a huge shift from roughly 2003-2008 and then almost no change since then.
For example, here's the 2008 cast of
Saturday Night Live:
These are hip, youngish people who try to be out there with their fashion choices, but everything still pretty much works today, except for Jason Sudeikis's blazer-over-polo-shirt-over-t-shirt getup, which we'd now consider to make one look like a tool.
Edit: fixed photo