One of the things that bugged me about bases in PlanetSide is that they're DESIGNED TO BE CAPTURED. Given two forces, the one on the inside of the base doesn't have much of an advantage over the one on the outside of the base. In fact, the folks on the outside are more likely to have vehicles, giving them an ADVANTAGE over the freshly spawned infantry types trying to hold the walls.
(edit: In retrospect, while the initial wave hitting the base is more likely to have vehicles, as soon as the battle gets passed that point, folks start spawning in AMSes and nearby towers to zerg their way in... meh).
OTOH, turning all bases into death traps won't be much fun for the attackers. They'd need massively superior numbers/firepower to take a well-designed base from those defending it.
Solution:
Make the Solid Defense portions of a base destroyable/deployable. Suddenly not-necessarily-accurate-but-powerful artillery is Very Helpful when trying to attack a base. You need to blow some chunks out of the walls to expose those otherwise great fighting positions. Infantry-carried explosives to breach walls.
Give the walls a ROOF. But one that can be removed with the application of sufficient explosives. Mossy/reaver strafing runs aren't going to work so well until some bomber/arty scrapes away some of their cover. Reaver rockets might do the trick in quantity.
Infiltrators with ACEs (or PS2's equivalent) could sneak up to the wall, plant charges, and detonate them from a Safe Distance.
As far as improved deployable defenses goes, there's a Historical Precedent called
Hoardings. Fireproofed wooden sheds attached to the outside of a castle wall during a siege to allow defenders to attack directly downward. Here's a (large)
piccy. The near tower and walls have hoardings installed, the far tower does not.
The simple/ancient arrow slit would be a vast improvement over the railings PS's bases have. A sniper could still tag you through one, but massed fire would have to actually chew through the wall first, or get Very Lucky.