Quote fromI think the shared abilities will consist of abilities that allow hybridization of roles in some sense. For example, I think the shared skill tree may have some emergency healing powers that the Gunslinger could have access to. Or perhaps the SW tree has some tanking abilities the Marauder can get. Etc.
The shared skill tree are talents/abilities that are the "core" of that class. So abilities most likely similar to those found at early levels and continues to max. My suspicion is that it would act similar to how WAR does their mastery system. As you level and gain points you put them into a tree. Every few points added you gain a new ability, but un-like WoW you had not have to spend multiple points in the same talent to make it better. Instead "these Core Abilities are part of this mastery. Each point spent increases their power" the core abilities you have from level one (Or close to it). Just a thought, and you can see how it works here: