Here's my opinion on the opening topic. I'm an East Asian minor and we went through a lot of these in class
Ancient Chinese culture and Japanese culture are very similar, especially during the Tang times. Japan imported Tang culture wholesale, from architecture to art style to language (not just written Chinese, but a huge vocabulary of kango) to cuisine to philosophy to government, etc. One has to realize that China throughout its history undergone major changes, two of the most fundamental ones occurred after Tang (one during the Yuan dynasty when China was conquered by the Mongols and the other during the Qing when China was conquered by the Manchus). During these periods, many cultural changes took place. For example Chinese switched from sitting on mats to using tables and chairs (although this occurred before Yuan but still due to external influence), started wearing outfits influenced by those worn by the nomadic peoples who conquered them (both voluntarily and forced), and changes in language in pronunciation also took place. (if you compare Middle Chinese pronunciation with Japanese pronunciation of many sino-xenic words in Japanese language, they are practically identical).
Japan, on the other hand, thanks to its geography, was immune from these changes from the outside (after Tang and before the arrivals of the Europeans). It preserved Tang culture extremely well (many of the Classical Japanese architecture we see standing in Tokyo and Kyoto today are typical tang architecture style, like the
Todai-ji). Japanese still sit on mats, still practice
Chado and
Ikebana (arts of the tea and flower, both Tang practices), Japanese people still enjoy cherry blossom, a hobby the Nara elites borrowed from Tang China, but modern Chinese no longer observe (I think, correct me if I'm wrong. At least it's not well known...).
With time, Japan and China went on very different trajectory. The latter's culture undergone fundamental changes with its turbulent dynastic cycles, while Japan not only preserved the Tang culture, but added many native touches to it starting from the
Heian period, during which we saw a strong assertion of native wa practice, like the
waka (wa poetry) over
kanshi (Chinese poetry).
So the result is that today these two cultures appear to be very different other than some obvious links (Confucian ethics, written language, sino-xenic vocabulary). But this is more because of the changing nature of the Chinese culture itself, rather than inherent differences between the two cultures.
Of course, I only addressed the issue of traditional culture free from Western influence. Today Japanese culture is greatly influenced by Western culture and has been for more than a century now, so naturally it's very very different from China which has undergone a dark age under Communism that not only delayed its development but also dealt a lot of damage to its traditional culture.