I wanted a simple thing: install some packages to support my wireless card. It seems I need a non-free driver for that, which is kind of disappointing; but that is life.
In some earlier experiment, I had installed the latest Linux kernel from the unstable archive (I am running stable on my laptop). No big deal, that: I have done it before on earlier Debian releases. There is always a small risk (both in terms of stability and of non-declared dependencies) but one can always decide to go back to the old kernel if there are problems.