Well, actually that is a very easy question. The answer is: everything.
But I'll try to point out some things I especially look forward to.
1. Finally ( finally !) getting out of that cave.
2. Seeing the story progress.
3. New Shenmue environments.
4. The music.
5. The characters. Seeing some of my favorite video game characters of all time again. Ryo, Shenhua, Ren, Lan Di, Xiuying. And finally getting to know some new characters: Niao Sun, Ming Ming, maybe Tentei.
Man I have to stop writing. Can't wait for S3 to come out.