Source: Bill Marsh
D3 2.10 features more sophisticated transformation and transition options and custom line and path interpolators. Map thresholds for colour gradients in diagrams, maps and similar graphs, as well as support for the L*a*b* and HCL colour spaces make it easier to work with colours. Simplified transform interpolation uses d3.tween to let the developer take better control of translations.
For dataset manipulation, users can now use TSV (tab-separated values) files as a database and make use of multi-value maps, which allow assignments of multiple values to selected elements with a single method command.
Variable padding in bar graphs combined with scale mark positioning should help with smoother bar graph design. Finally, the new static localised date formatting allows D3 to be built with static support for time formats in, among others, Russian and French.