- Having a typesetting quality and control like TeX does, including mathematical typesetting, although improvements could be made to that, too. (I think I would also like the syntax more like TeX has)
- You can include PostScript codes within the document which can run during the typesetting process (rather than only during output), therefore allowing it to affect decisions of page breaks, etc, as well as allowing PostScript to draw diagrams and control text rendering. (I had written a PostScript program to load PK fonts, so that would make it possible to use PK fonts from TeX as well.)
- Full support for non-Unicode text (without converting it internally to Unicode). (It is OK if it also supports Unicode as long as the code to support Unicode is avoided when not using it (in the entire document or in a part of it).)