Thanks for this very clear explanation and guide. I've used LM for about three years and LMDE is an entirely new experience. I find myself spending much more time here in the garage with you-all. (Yes, LMDE is the garage-band version of Linux Mint.)If people pay enough attention to what they are upgrading this is pretty true.... Where LM_Ubuntu-base seems intended to hand hold, LMDE does not.... Many who come to LMDE do not seem to be beyond the point of wanting a rock solid, stable, pre-installed and configured device placed in their hands. Wanting such is ok, just choose your distro accordingly....
Your excellent comment tells me that what I was looking for with LMDE, i.e., Debian Lenny stable with a Mint face, is something I'm not likely to find - and haven't. There are just all these minor issues - suspend that doesn't work on my ThinkPad X32 laptop, fleeting problems with wireless (solved), and the constant feeling that the next series of updates may break something.
I really want a stable distro. The alternatives seem to be Scientific Linux (RedHat-based and reputedly very stable) or Mepis, the latter I don't trust based on past experience - like LMDE, I found that with Mepis regular tinkering is required. And with Scientific Linux there's not much hand-holding.
Now I'm wondering if installing Debian Squeeze would be a good idea. Difficult in the beginning, but trouble-free going forward.