One of the things that I always love about manga is how much intertextual references are made between them. When you have that knowledge, or cultural capital, it makes the reading experience more interesting and it takes the reader to a new level of understanding. It is very easy to convey a feeling or importance through what could be a subtle reference.
The whole intertextuality between xxxHolic and Tsubasa was a little confusing at times and I still don't feel like it was completely explained in Holic, but it was very interesting to follow it through till the end. I haven't read another manga that relies that heavily on intertextuality to establish a plot before.
