The announcement of Planetside 2 has piqued my interest.
I have a lot of fond memories of that game - thought I admittedly quit after the Core Combat expansion nobbled it badly. As a Terran Republic player I experienced the joys of being a member of a dedicated Prowler tank crew, moonlighting as a engineer/infiltrator when solo, and eventually linking up to a 15-man squad. And yes, I was geeky enough to get involved in practise runs to capture areas as fast as possible.
Firstly the good stuff. Mass three-way battles (when the servers could handle it) led to fluid battlelines, sieges, and sometimes entrenched warfare. Towers and bases (which contained the in-game spawn points) changing hands could decisively swing a battle - whether taken by a full advancing division or a set of sneaky infiltrators. The skill system allowed you to change your character build so you could do what you wanted - a nice bit of versatility that still seems better than the standard Assault/Heavy Weapons/Medic etc. A favourite combo was the infiltrator/engineer/Mosquito pilot, sneak behind the lines, take that unguarded tower, set up turrets, then switch into a MAX (armoured) suit and defend.
Now the bad stuff. Weapon/vehicle balance was a bit off (for those who played, the term 'Magmower'sums it up), you often had to 'follow the zerg' to find the active combat, and often by the time you got there the area had been mopped up and the troops had moved on (action was fast, and the main map did not update very quickly). With the introduction of Core Combat, a chunk of the game died, in my experience. Players were distracted from the battlelines to the cave section, which if controlled gave major benefits to aboveground troops, leading to an endless skirmish in a single area. The players needed to take said caverns in order to push forward on the surface simply because the forces had been drawn into the caverns battle in the first place, leading to a loop. And a crowded laggy play area.
As for the BFRs (mechs), you'll have to ask others. I suspect the game balance was probably just battered further.
So in summary - before the new content wrecked things, Planetside was perfectly good MMOFPS. Bad development choices and poorly thought through extra content unbalanced it. It had its moments of greatness. At the very least, a suitably tarted-up version could do very well.
I hope they can get it right this time - if done correctly, I'll be re-enlisting with the TR for a second tour of duty.