WOW, first off let me say WELCOME BACK! lol
So much has changed, it feels like everything has changed. Basicly here is the synopsis...
ATMAs are better than gear, they are amazing! (some of the better ones drop off of NMs in Aby) See: http://wiki.ffxiclopedia.org/wiki/Atma
However in-order to use ATMAs you need Lunar Abyssites (http://wiki.ffxiclopedia.org/wiki/Lunar_Abyssite)
Exp is mostly done solo pre level 75ish (30 by some accounts) and in Aby post level 75ish. Aby exp is the best with Dominion Opps from my experience (Uleg, Grab, Altepa).
The new must have gear is AF3 (+1/+2).
Abyssites are also very nice for supplemental reasons http://wiki.ffxiclopedia.org/wiki/Category:Abyssite
Everything else is mostly optional. I'd recommend hooking up with some old friends and/or an Event LS to assist you.