Bitcoin features that were experimental. This includes: using IRC to find peer addresses (in irc.cpp and irc.h), marketplace/product/review (market.cpp and market.h), and sending bitcoins to an IP address. Code related to GUI.
Other tips
Just get started. Don't try to read a lot of theories about bitcoin before you start. The code is pretty clear. Only look up an unfamiliar concept when needed (like merkel tree).
Link to the codebase
https://github.com/bitcoin/bitcoin/
Which release/version did you read?
0.1.5. (https://github.com/bitcoin/bitcoin/releases/tag/v0.1.5)
Why it is a good project to read?
Starting point
ui.cpp (https://github.com/bitcoin/bitcoin/blob/v0.1.5/ui.cpp). This is where the initialization happens when a bitcoin application starts.
Reading order
Interesting parts
Almost everything. It was an innovation.
Any parts should be skipped?
Other tips
Links and resources