Upgraded to 3.4 yesterday and nothing but problems! Ugh.
The first was that suddenly it is displaying "space" where forms have hidden fields. Huge space! Solved that by enclosing the hidden fields in a div with display:none style. Time-consuming. Thanks for that.
At least I could work around it.
Now I can't upload images! At all.
I do the upload thingy, it does its crunching thing and then shows the usual screen of details, options, etc for an image, but there is no thumbnail. If I copy the link and paste it to my browser, I get a 404. So I delete - which it does without a "can't find that" error message - and try again. And again.
I have tried uploading using the Media Library, and also by trying to upload to a single post. I have tried using the multiple file uploader as well as the single file browser uploader, all with the same results. No idea how to work around this one.