Since YouTube start serving 1080p videos by using Adaptive Stream, and it will separate video and audio into multiple files, so we can't download 1080p videos directly. Let me know if you still want to download these separated video and audio files, I will upgrade my website shortly. But I think that we're no way to download the 1080p videos directly in the meantime (sadly).
Update: You can download the separated audio and video files from now on, you can download them and merge into one video file if you need it.
2013/11/07 update: 1080p 直接下載測試中，若您有遇到任何問題，歡迎在這裡留言給我，謝謝 1080p direct download is under testing, you are welcome to contact me if you have any question.