This is more-or-less based on the actual heroes in the Hall of Heroes, and how each of them has a "class" that embodies them. The idea is that each armor and weapon would now have an added classification to them. Things like Gunslinger, Striker, Defender, etc. The idea is that the more of a specific item class you wear, the greater your benefits.
The first three levels would provide decent passive abilities. Defenders gaining a boost to shield health and overall defenses, strikers gaining mild movement and attack speed increases, that sort of stuff. But when you're using all four pieces of an item that would be considered part of the same group, you gain a Greater Passive. An example is for Defender: Your Greater Passive is an augment to your shield which increases the radius of your shield block and will also shield allies near you, similar to a Guardian's shield in Lockdown.
Gear classes are an iffy boat. Certain "classes" in the Hall of Heroes don't have full sets (Demo and gunslinger have no shield, angelic and chroma have no weapons or shields, all the specialized damage defenders have no weapons, etc) which makes this a bit harder to approach.
Personally I feel the current balance is fine as you can mix and match gear without feeling like you're being hampered and with full bonuses you're essentially "playing that class". Playing around with more dynamics for each armor set would be more interesting however, such as status vulnerabilities or bonuses (shadow defender having a random bomb buff, piercing defender having a random sword buff, etc) might be cool.