Key features of i3 are correct implementation of Xinerama (workspaces are
assigned to virtual screens, i3 does the right thing when attaching new monitors), XrandR support (not done yet), horizontal and vertical columns (think of a table) in tiling. Also, special focus is on writing clean, readable and well documented code. i3 uses xcb for asynchronous communication with X11, and has several measures to be very fast.

Please be aware i3 is primarily targeted at advanced users and developers.