Lots…!
Many useful LaTeX packages have not been implemented, and those that are aren’t necessarily complete.
Contributed bindings are, of course, welcome!
Low-level TeX capabilities, such as text modes (eg. vertical, horizonatal), box details like width and depth, as well as fonts, aren’t mimicked faithfully, although it isn’t clear how much can be done at the ‘semantic’ level.
a richer math grammar, or more flexible parsing engine, better inferencing of math structure, better inferencing of math meaning…and thus better Content MathML and OpenMath support!
Could be faster.
Easier customization of the document schema, XSLT stylesheets.
…um, …documentation!