A few example of good cross class abilities:
Sentinel on any class is about 15-16 secs on 90ish% dmg reduction.
The attack buff on other melee classes
Cure/sacrifice on other classes
feather foot on other classes
emulate on other classes
Glad's accuracy boost
Invigorate from lancer is a must have at nms and such.
Honestly I see no reason to use any dmging abilities from other classes, but the support ability are really powerful when you combine them with other classes and the affinity boost them up more.
