- Multi-rename tool.

- Synchronize directories (symmetric and asymmetric, with subdirectories).

- Background operations.

- Operation queue (so that you don't thrash disks while doing many operations).

- Start menu.

- Displaying directory sizes easily.

- Great file search.

- Diff viewer for text files.

- F3 quick file viewer.

- Compare directories.

- Plugins.

Those are my favorites.