Plain TeX (which is what KerTeX offers by default) doesn't seem that complex, but it does have the disadvantage of not offering much of a separation between format and content. Both LaTeX and some of the more sophisticated troff macro packages do a better job of allowing "structured" editing.
Also, what level of font support is available in KerTeX and Plan 9 troff? I'm assuming that neither offers the level of "plug and play" support for modern Opentype fonts that can be found in XeTeX and Heirloom troff, but how are they otherwise? Mike
