[szhorvat]

For practical purposes, nothing. If you have used Mathematica in the past, and know what it is: the "Wolfram Engine" is basically the Mathematica kernel.

Note that this free version only includes the kernel, not the notebook interface. But there is nascent support for Jupyter, so productive use is very much possible.

"I'd expect that many of the things I'm used to doing in Mathematica wouldn't work here"

What's amazing is that this isn't true. There are no _technical_ restrictions. You can't do Manipulate because the notebook interface is no there (or rather: it's there but it's read-only). But all the computational power is available.