Shadow has a lot late game defense potential. Above Duelist for sure. This is due the fact Shadow has access to Energy Shield nodes from the Shadow and Witch side.
As a defensive system Energy Shield often out scales Evasion [the worst], Armor [power early/weak late] and Life [limited]. Most late game Shadows go Chaos Inoculation currently. However, even with the changes to CI in OBT ES remains a strong defensive system for hybrid Life/ES builds. There are 2 easy clusters of Life and ES for the Shadow which are early in the tree as well.
Every class can do every thing and essentially reach/use the same nodes/skills. The only real impact class has on the game is starting point.
A Marauder [Barbarian] can very well use a bow, sword, wand of cast spells like a Witch [mage]:
http://www.youtube.com/watch?v=CqwdiOdYfkI
Where as a Witch could actually be full melee tank, spell caster or double dagger backstabber:
http://www.youtube.com/watch?v=_aP9O0Vnt0c
Rangers can be masters of 2 handed weapons, bows or 1h/shield:
http://www.youtube.com/watch?v=gqvE66YFRO4
Class is all about where you start on the tree and how fast certain skills/nodes are accessed.