It's true that Paladin does not fill the role that it once did, but I happen to find that the job is a lot more fun to play now that it doesn't. Now, let me first say that Paladin isn't my main job, and I don't play it as often as I do other jobs, but it does have a special place in my heart as a fun job to play.
First of all you should familiarize yourself with Abyssea, because a lot of the gear that you will be wanting, as well as pretty much all of your experience points will be coming from there. There's a lot to absorb on how it works, so get going on it ASAP.
Next, realize that you're probably not going to be tanking generally anything in abyssea at all unless you've got incredibly epic gear and skillz, and great support. People use THFs MNKS and DNCs to tank there because they can dish out ridiculous amounts of dmg to keep hate and they have near perfect damage mitigation, requiring very little in the way of support. in the face of that, PLD doesn't really stack up.
I've found that once I got good DD atmas, a pretty epic DD set, and a Badelaire +2 for the Chant du cygne WS I can dish out a pretty good deal of damage. I'm not the best DD on PLD, I know, but you can surpass just about anyone with mediocre gear/atma if you put the work into getting the gear. In abyssea for experience points no one cares what job you are so long as you can hit things and toss a few cures when needed, just be aware that you're not going to be tanking. They may as you to flash a mob off the puller, but there's really no way you'll be keeping hate with an entire alliance of people doing their best to top each other in WS dmg.
I'm pretty lucky in that I both have many other jobs to choose from to fit the needs of a group going after NMs and stuff, and a social LS that lets the PLD take a crack at tanking some of hte bigger stuff every now and then. And we use PLDs for a lot of things outside of abyssea like KSNM99s, gods, older HNMs etc. PLD can still do very well as a tank outside of Abyssea.
Realize you won't be tanking pretty much anything in abyssea.
Familiarize yourself with Abyssea ASAP.
Try to get good atma.
Build a good DD set of gear.
Have fun, don't let anyone tell you not to play your favorite job because they think it's worthless. PLD can do quite a few things, including an impressive array of red and blue abyssea triggers, backup healing, great DD with the right gear/WS and it can still hold it's own as a tank outside of abyssea, and inside with the right support, though not as efectively as other jobs most of the time. You can also solo quite a few things pretty effectively if that's your thing.
90DRG 90DNC 90PLD 90COR 90RNG 90THF 90BST 81NIN
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In my day vampires sucked blood... not ****.
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I killed Asmodean, let the speculation end.