An irritation with current GUI interfaces
Why are programs so convinced that I have horizontal real estate to give them? Everything seems to want to put important information in sidebars and panels and so on (usually non-optional ones).
I don't know about other people, but for me horizontal screen space is more precious than vertical screen space. If you eat vertical space all that happens is that I get fewer lines of your real content (which is still irritating; please don't). But if you eat horizontal content, your application gradually takes over my entire screen, because I don't have any space left to put things besides it.
(It's no wonder that running one program at a time in full-screen mode is so popular.)
I don't know where this design approach comes from, although I suspect that some of it is a lingering remnant of MDI (itself one of the more wretched and arrogant ideas in GUI interfaces).
If programs have to have this sort of interface, I wish for two things: the ability to completely close the sidebars (many programs seem to give them a minimum size) and the ability to rip the sidebars off as separate top-level windows, so I can park them somewhere more useful. Programs earn bonus points for remembering how big I had sidebars before I closed them and then re-opening them at the same size.
(This rant has been brought to you by the current interface of Evolution, which glues a list of accounts and folders to the left side of the content window, even when all I am doing is reading email from one folder and it is a complete waste of space.)