Multiple chat panes
The chat monitor is quite good if you're mainly interacting in one channel and passively monitoring others, but it's not sufficient for cases where you're interacting with people on two or more channels. I am very frequently engaged in simultaneous conversations across multiple channels, and the current interface is unfortunately very clunky for this kind of use.
As such, the ability to have multiple channel panes displayed in a window with the ability to easily change the focus of the message entry box between them (i.e. good default keyboard shortcuts for it + click to focus on the windows, say) would be extremely useful. This kind of functionality does exist in comparable IRC alternatives (e.g. the common screen/tmux + irssi combination), and it is by far the single biggest feature I miss from them.