Gmail has been redesigned too, and it's just as bad. When you reply to a message, less than 40% of the vertical space is available to, you know, actually write your message. Look at this screenshot:
Gmail, while replying to an email. Note how the actual writing area is just a small window in the lower half of the screen.
So why am I not annoyed? Why am I not abandoning Gmail? Shaking my fist impotently against a cold, uncaring sky? Because It's All Text. This is an add-on to Mozilla that lets you edit any web page input using your own favourite text editor.
Look at that screenshot again, and note the yellow-green circle? There's a small button there that says "Edit". Click on it (or choose It's All Text in the right-click menu) and your editor will open with the current contents of this text input area. Write away, in full screen, in your favourite tool. You save the text and the plugin automagically inserts it right into the web page, as if you had typed it in yourself.
This is wonderful. I think you can use almost any text editor you want as long as it can save as normal text (no word-processor, obviously). I use Gvim, but you can use Gedit, Notepad, Textmate, Emacs or whatever you want. Works on Linux, Windows and OSX (though there's apparently a few extra steps for OSX). It's not just the convenience of editing in a real editor. You can save drafts or copies of what you write, and if you leave the editor open until the text is posted your text will be safe even if the website manages to lose your input. If that happens you just reopen the input page and paste your text into the field - no more rewriting your entire text.
The one place it doesn't work right now is on Google+. A few other web sites manages to lose the "edit" button in the corner, but It's All Text is still available and working through the right-click menu. But on most websites it works just beautifully.