Hide Tab Bar
I can't really say for previous version as I didn't pay attention to this issue, but in OS X Mavericks it is not possible to hide the Tab Bar.
Apps like Safari, Terminal, and now Finder have tabs and a menu item in View called Hide Tab Bar (⇧⌘T) which is always grayed out. In fact it only works in the case when only one tab is present, and the Tab Bar is forcibly shown using View->Show Tab Bar.
I see this as a bug, and it takes away some of the productivity focus of the Full Screen mode in OS X.
What do you think? And does anyone have, or know a solution?