Yeah, the limechat default is ugly (with the two vertical panes) and standard irc courier-1980s-look. And right, it doesn't have tabs--and if that's the deal breaker, then that's cool with me.
I'm just really anal about polished looking software on my computer. For the screenshot you posted:
1. The aqua status indicator on the bottom left and on the userlist.
2. The line just below the titlebar (the topic bar), with two white pixels on the very left and right.
3. The line along the very bottom, going through the tabs.
4. The 10.1 era aqua arrow button to the right of the text entry field.
5. The text inside the tabs aren't even properly spaced proportionally.
6. The ugly twin blue bars on the bottom right (not sure if this can be disabled, can it?)
But other than that, you won't get xchat looking as clean and polished as the screenshot I posted since it doesn't support message styles/themes. That's all I really ask for, because changing the font and colors just isn't enough really.