What are the core features that must not change?

[DviddLeff]

While answering the servey I had in my mind a few things that I could not let change about PS; the main things that PS offers that make it what it is and make it unique.

These were:

1. Multiplayer battles with 100+ on either side.
2. FPS controls
3. A levelling system that merely gives players more options, not artificially makes them stronger and do more damage. This allows everyone to play together, no matter their experience.

That is it, everything else can change as far as I am concerned but those three features make PS unique and appealing.

Thoughts?

Any other really important main features that I missed?