• The text is automatically reflowed to the size of the window
• No concept of "pages" at all, the text in a chapter can be scrolled through continuously
• The reader that I use (fbreader) appears faster on my laptop and uses less memory than PDF readers such as Okular or (horrors) Acroread
• Files are small (<500kB for an entire book)
• Foreground / background colors can be configured, this is nice if you prefer reading on a dark background

It's truly post-paper. PDF: The 15th century called and wants its fixed sized white pages back.

Recommendation: fbreader is a nice program for reading epubs on Linux. It can be found in the Ubuntu package repository.