I've recently been doing a little work on implementing a few LaTeX kernel bits in ConTeXt, to simplify porting LaTeX code over. It's now at a point where a few of the pieces might actually be useful, and certainly to a point where some comments would be welcomed, so I've put up a version of it online, in this directory:
What you'll find there is a fair number of module files, along with a couple of "latextest" files that I've been using to try out the results. latextext.tex is to be compiled in ConTeXt; latextext-latex.tex is to be compiled in LaTeX for purposes of comparison.
In any case, I think the implementation of \newcommand (and its relatives) is about up to "beta" level. Ditto with LaTeX lengths. And there's a working LaTeX-like counter implementation, based on ConTeXt counters underneath. (It's not yet hooked into ConTeXt's counters for footnotes and equations and pages, though; I'm not really sure how to do that.)
Also, environments are working, in a sort of hackish way. The "array" environment works, but again a bit hackish; I'm planning to clean that up soon, so both of those will get major revisions.
I'd particularly appreciate any comments on the \newcommand and counters implementations -- those are in the t-ltcmds and t-ltcnts modules, though \newcommand also depends on t-lterrs and t-ltbase.