One of the best parts, in my view, was Zeljko Vrba pointing out, in effect, that Postgres both embedded scripting languages and provided an interface that allowed it to be used as an extension to a scripting language. This provides a wonderful dual example.
Taco and Hans already stated their opinions, and I won't speak on their behalf, but I read the entire thread on c.t.t. and would like to point out that in the discussion, Željko seems to say the exact contrary to what you report here: Postgres embeds Python as a scripting language -- just like LuaTeX embeds Lua -- and does not offer the database engine as an extension: it only gives access to it through an API, and does not extend Python with a database module. I'm quite sure there would be ways to do the same with LuaTeX (provided one knows what sort of result one expects from it exactly), and David Kastrup even described possible solutions, which Željko mentioned in his very same post. You did not care to comment about it. Arthur