I don't think I have EVER been so disillusioned with a WordPress upgrade as I have been with WP 3.5.
It seems that WordPress broke the some most important rules when it comes to upgrades -- KISS and documentation.
First, there was NOTHING wrong with the media uploading in the previous versions, yes someone thought it needed to be changed. But what they didn't take into consideration was that many of us have been using WP since version 1.5 and have been upgrading each time a new version has come out. Our configuration files are so old that there are spider webs in them, and we still carry a lot of the legacy code through each previous versions.
So now Automattic came out with what they thought was a really nifty change in how we add media. Groovy -- I am sure it IS really special. However, they fail to take into consideration that we, who have been around since the beginning, still have a lot of old code that never gets washed out with each upgrade.
So here I've been, for the past ten days or so, dealing with not being able to upload media on my media-intensive website. Why? Because the fine people at Automattic didn't think about a lot of us with old legacy code in things like our configuration files.
I hate to sound like I am complaining, but hey -- WAKE UP SOMEONE!
If it ain't broke, don't fix it. If you "fix" something, at least make it work for us or tell us what we need to do as part of the friggin' upgrade. Don't assume that we are going to eventually figure it our ourselves -- most of us aren't that adept -- and frankly, we shouldn't have to search high and low for a solution to something that YOU changed.