New features since 3.0:

Edit box buttons and line splitting can be hotkeyed
Split editbox which shows the state of the active line when it was first selected
File → Open Autosaved Files... to open previous versions of the current file
The clip visual tools update all selected lines
The Effect box lists all effects used in the file, similar to the Actor box
The subtitles grid can be scrolled farther past the end of the file
Words can be removed from the spellchecker dictionary
Much better downmixing for >2 channel audio