1. With the calc package, all \setcounter (etc.) calls accept integer expressions such as (1+6)/2. Of course, similar (and more efficient) results can be achieved in eTeX with \mycount=\intexpr(1+6)/2\relax

2. \setcounter and friends are occasionally redefined inside certain contexts. In tabularx, for example, material is typeset multiple times and \stepcounter is redefined to only increment the first time the code in the table is executed:

\documentclass{article}
\usepackage{tabularx}
\begin{document}
\newcount\mycount
\newcounter{mycount}
\begin{tabularx}{5cm}{X}
\global\advance\mycount 1\relax \the\mycount \\
\stepcounter{mycount}\themycount \\
\end{tabularx}
\end{document}

This counter redefinition is usually a good thing, but I once had an unfortunate interaction that caused a bug (although I forget exactly how…)

\mycount\somemacro \morecsnames

which is usually where problems appear. As I said, LaTeX uses \relax internally.

I’ve mainly focussed on looking at things as numbers, rather than the entire label system in LaTeX. Things get complex, as you say.

Joseph

Personally, I use a space instead of \relax, but I guess this is a matter of taste.

And one more thing: there is also \refstepcounter, which is used for counters which number something that should be accessible by the label-ref mechanism. The details of label printing are a bit tricky, though.