This seems like a pretty obvious bug, and I'm surprised it wasn't fixed instantly.
When you go to insert a hyperlink in the body of a post the link window will pop up at the far lower left of the screen. Then once you input the information the window will not close and save the changes.
Considering most posts have a number of hyperlinks that mus be added this is a HUGE bug.
I have tried this in Mozilla, Chrome and IE and it does the same in all of them.
Now I have to figure out how to successfully regress to 3.4 without messing up my site. no happy, Not impressed. I expected more from WP at this stage of the game.