Easy and flexible for beginners
End users who aren't programmers should be able to have an extreme amount of control over their journals without ever seeing or knowing HTML, CSS, hex color codes, or other web geekery. The styles and themes should have graphical previews and graphical wizards to customize every color, option, and text.
Powerful, familiar, and powerful for geeks
The core of S2 is actually a programming language, with syntax and semantics resembling Perl/C++/Java/etc. People who do know how to program will be able to do anything they want, including make styles which all their non-geek friends will able to use, since the styles are capable of reflecting all their options, which the graphical wizard lets them tweak.
Developers shouldn't have to write a lot of repeated or unnecessary code. Authors of layouts need not override all functionality, instead being able to fall back on the core-provided functionality.
Rather than letting users program directly in C or Perl, S2 ensures that the resultant compiled code is both safe and fast.
S2 code can't get access to anything on the host machine or do anything malicious.
S2 code can't waste host machine resources. (CPU or memory)
Internally, S2 code is compiled into a lower level language and run directly when the page is loaded. Unlike S1, no parsing is necessary at run time. Also, S2 outputs directly to the client instead of being entirely buffered in memory first like S1, since S2 doesn't need to do tons of templating replacements at the end.
Popular S2 code & data is cached by the webserver.
S1 was English-centric. If you wanted to say "1 comment" vs "2 comments", you used the magical %%s%% which was an es when plural and nothing otherwise. But then of course we had to make a magical %%ies%% for "1 reply" vs "2 replies". This ignores languages where the plural form is entirely different, or there are multiple plural forms.
S2 uses UTF-8 everywhere, handles plural forms correctly, and makes customizing text/date formats/etc all trivial.
S2 makes it possible for languages to override not only the text which will appear in the final output, but all the property names and descriptions as well, so understanding English is never a requirement to customize the look of one's content.