Lots of nice touches make this the best Twitter client I've tried. Excellent keyboard navigation. Has a keyboard shortcut for shrinking URLs. Has lots of sensible configuration options (like color scheme, how to "Copy" tweets, hotkey for activating the app). You can view the error console. You can define tabs that filter your timeline with a regex.
For all the talk of how Twitter clients have stimulated UI innovation, I haven't seen much that impresses me. YoruFukurou stands out for all its nice little touches.