Virtually every (useful) passive boils down to "deal more damage" or "be better protected". Sorry, not much depth there. The only difference be classes' passives end up being who gets more damage, who gets more protection, and who gets the shaft.
As in every combat-oriented RPG. When game is about dealing damage while staying standing, every skills or weapon or gear either makes you deal more damage, or avoid being damaged.
Also, most bonuses from skills are conditional. And you have to figure out how to make that condition fulfilled when you need.
For example, Zero has 1 shot 1 kill skill, that give huge damage bonus, but only to first shot in magazine. To get most of it - use guns with big alpha, such as quad shotguns or snipers. Which means you will have to reload a lot - so you'll need Killer, Fast Hands, and possibly At 1 with the gun, if you want to focus on Sniper. But Killer, for example, works only after you kill someone. What if one shot was not enough to kill? Then you take Killing Blow to finish off enemy with melee if needed, and Be like Water to capitalise on mixing melee and guns. Of cause, it only works in melee range, which means that your gun is a shotgun. And you shotgun then should gave a baginette to amplify your melee. Now, as you already rely on melee, you can as well take other things in Bloodshed tree. Then you can blade-critical-backstab people in stealth, and when your Many Must Fall chain is over, you can go back to shotgunning.
Another example - Maya's Converge and Ruin combo perfectly, Slagging, DOTing and even stunnng for a couple of seconds in area. But then there is a question - are you going to milk maximum from Phaselock, or you want to be not as relying on it? In first case, you want to have skills that give bonuses during phaselock, such as Wreck. Also you can use the fact that the guy you phaselocked stay in place, so is easy to hit. So, take Chain Reaction and he will "share" his pain with the rest of the crowd. You can just consider it a nice bonus, or you can combo it with a Maliwan SMG and Flicker and get near-guaranteed DOT on everybody in big area. But it all relies on Phaselock, so if you going to fight with something immune (such as boss), you have to use something else.
On the other hand, of cause, there is not that much good combos, and they start really kicking in at around middle of second playthrough. So, many people just get bored way before.
Only snipers can reliably hit critical points at distance now. On the other hand, you have much better mobility now (with low-grav jumping), so you can just close in and apply anything with high DPS directly to the forehead.
