Debian does not have long term support
Every so often someone says that Debian's stable releases have a long support period. Unfortunately this is what one would call 'wrong', and for at least two reasons.
(For now, let us define 'support' here as 'gets security fixes'.)
First, it is wrong as a plain matter of fact. No Debian release has ever been supported for more than a sliver over four years, and only one release has hit that mark (Debian 3.0 'woody', released mid 2002 and supported through mid 2006, per here). Every release since 2005 has been supported for only about three years. Ubuntu LTS manages five years; Red Hat Enterprise Linux goes even longer.
Second, it is wrong as a philosophical matter because Debian doesn't promise any particular support period. Debian doesn't promise to support a release for X years, just more or less to support it for a year after the next release comes out. If the next release comes out in roughly two years (as has been the case since 2005), you get three years. If the next release comes out in a year, you get two years. And so on. The only way you get long support periods is if Debian is painfully slow to make releases.
This has two consequences. First, Debian support periods are unpredictable. If you install a machine with Debian, you have no sure idea how long you'll have support for (although you can often make an informed guess). Second, the real support period for a machine can be as low as a year, if you have to install a machine shortly before the next release comes out.
(In theory the minimum period is even lower, but this would likely require Debian to do two releases in a year. This seems, well, unlikely.)
Real long term support involves three things. First, you must commit up front to a definite support period (as Ubuntu and Red Hat do). Second, you must actually have a long support period (which is always shorter than it looks in practice); three years doesn't really cut it even if Debian committed to that for releases (which they are not going to do so). Third, you need a significant support overlap between the current release and the previous release because of the real support period issue.
Debian does none of these, which is fair enough; Debian doesn't claim it has long term support. I just wish people would stop claiming that it did on its behalf.