The ability to split the main window pane horizontally and separate server messages and commands from normal chat is beautiful. I don't know why all IRC clients don't do that. Really what sells me on this over Konversation, which is very clean, or Quassel which has a very nice chat monitor feature where you can split the window horizontally and have a pane that pulls just the chat from all channels which can be nice with how dead IRC can get. Aside from that it's nicely customizable and just works. Also it works with SASL and even has a setting to auto disconnect if SASL fails.

KVIrc does not need to be installed. It can be kept on a flash drive and run from there.

Help files and online resources show all of the feature and scripting capabilities.

Despite what's been said, development has been continuing at a steady pace on version 4.3.1 and more recently 4.3.2.