1) PDF viewer. 2) Flash. 3) Some audio/video codec support (e.g. Chromium has no support for MP3, AAC, H.264, or the MP4 container format; see <http://www.chromium.org/audio-video>). 4) Crash reporting, metrics, that sort of thing.
#1 and #2 are certainly closed-source. I can't speak to #3. [EDIT: as surrealize points out, #1 is no longer closed-source.]
From an end-user perspective, there's a significant difference between "browser that will play the music and the videos and show me the PDFs" and "browser that will not do those things". Chromium, while open source, is really not a viable end-user browser on today's internet.