I've been doing some research in preparation for next year when I will
be replacing my gas furnace and a lot of people seem to mention that
the make of the furnace doesn't matter as much as the quality of the
installation ... but what exactly does that mean?

The furnance comes preassembled from the factory more or less so it's
not like the contractor is building the thing. How can a poor
installation ruin a good furnace? What makes it a poor installation?
Are we talking about the preparation part of the installation, like
getting the correct size (BTUs), proper chimney size, correct vent
sizes, etc. ... or are those persons refering specifically to the
installation part ... what can go wrong during that part?

Thanks,
Harry

P.S. The more technical the information you provide the better ...
thanks.