The channelbar (where channels, queries, dcc chats etc are being shown) can't be moved with the mouse anymore. I know there is a setting for this in mirc options, but it was kinda nice to be able to move it with the mouse.
Personally I prefer it this way. Every now and then when I was trying to click a button at the end of the switchbar I'd accidentally drag and move it from the side to the top. Can't happen anymore (at least, not as easily).
Spelling mistakes, grammatical errors, and stupid comments are intentional.