MathML is not a typesetting system so there is a difference between what it can specify and what you can currently typeset with it. I usually typeset MathML by translating it to TeX for example, so necessarily that restricts things to those that are easily typeset by TeX.
Things that MathML has simple markup for which are (more or less) hard to render using LaTeX include.
Full range of Unicode symbols. (This is impossible to support in classic TeX, it becomes more of a possibility using
unicode-math and xetex or luatex.)
Automatic linebreaking. (This is more or less a requirement on the web, unlike traditional paper typesetting for which latex was designed) the author can not really manually linebreak if the reader is using an unknown device and screen size.
Stretching delimiters based on the size of the parent expression so all four brackets in
(a+b) + [X+Y] whatever a, b, X, Y.
Stretching delimiters while allowing line breaks.
Multiple pre and post scripts on the same base.
That list is just off the top of my head, I'm sure that other issues could be added.