There were a few things I loved about Firefall, both BT and AT (Before The9 and After The9)
1. Giant mechs (of course, i mean that goes without saying.)
2. i liked riding around in my lgvs and just feeling the breeze
3. You only had to create one character. Changing character customizations was free, you could access all frames on the same character, and you could do whatever you wanted, then switch to doing something else, all on a single character.
4. BT, there was no fussing about balancing skills for pvp, which left a lot of room for creative weapons and abilities.
5. Unlike most mainstream mmos, you could get npcs of different factions to fight each other. Not only is it a neat way to shake off enemies, it just made sense that warrinng factions hated each other enough that they would be at each other's throats, unlike games like SWTOR where (for the most part) npcs simply don't aggro each other.