Here I've tried to take the best features of my favorite irc clients and explain the rational.
You need to see the most commonly used features fast.
users
channels
active chat channel
common buffer (scrolling chat from all channels your in)
typing area
Users and channels in European languages go left to right so if you use tabs they squash up and become less readable. Much better to have vertical channels. These could either toggle or use slide-bars to get out of the way
Similar to ChatZilla with the channel tree plugin or Loqui
where you do get dead space then an option to combine user and channel in the same column like in loqui makes good sense.
Nothing worse than trying to scroll back through a single line of text to re-read a long sentence. Hence some sort of expandable typing area is required. Again either on a toggle or drag-able slidebar
The next thing is alerts for stalking/monitoring. Both visual and audible alerts are required with good customization options.
The option to turn on or off channel joins and parts on a per channel basis
