Differences between IBM thinkpad T and R series?

Can anyone explain the significant differences between the current IBM R
and T series Thinkpads? Aside from a slight weight difference and the
magnesium composite cover on the T series, I'm having a hard time
differentiating. Of course you can order them with different components
(hard drive; cpu; screen size/res.), but you also can order an R51 and a
T42, for example, with the same screen size/res; same hard drive
size/spindle speed; same amt of memory; same optical drive).

So, what are the differences? Heat dissipation (because the T series is
thinner); some difference in I/O stuff I've missed from reading the
documentation?; some durability differences (must be some reason why the
T series machines weigh less); other?