2.1 does not introduce new features with respect to 1.99.21, the software has been gradually refined with new features and bug fixes along the whole 1.99.x series of releases I think (I am following it starting from about 1.99.9).
Important features of 2.1 are a much improved LaTeX export filter, improved HTML export, a comment facility (for collaboration), improvements on plugin support for several programming languages (I did not test these extensively), a very nice mechanism for making it easier to use the label/ref mechanism (includes a tiptool for immediately seeing the referenced object).
