Baez - like most others - gives (IMHO) a quite unsatisfactory introduction to Einstein's equation. They all start with the equivalence principle or similarly unintuitive reasoning, relating it to special relativity and so on. But I did not find any introduction that gives me an intuitive description of what the Einstein equation actually describes.
Baez talks about $t$-momentum. What is a $t$-momentum? And what is a $t$-momentum in t-direction? And then what would a $t$-momentum in $x$-direction be? And what do I have to think of a $t$-momentum flow? He explains: "The flow of $t$-momentum in the $t$-direction is just the energy density." As if that explains anything at all.
IMHO a much better approach would be to describe simple experimental results and then show how to calculate them using Einstein's equation, maybe using a reduced and simplistic form initially.
