Welcome back, though can't say I knew you. As for what changed, there's a huge ass list. Mostly it'd benefit you to just go to SE's site and crawl back through the updates and see what all they release. That contains alot of the general information, and from there you can investigate specifics if it interestes you.
As for campaign, it's a bit of an odd ball. At first, soloing is more exp/hr then Campaign. But the longer you stick with it, the more rewards it can give you. If you get the highest medals, which isn't hard and only takes time, it can give you more exp/hr then soloing can...I think. I've never been keen on soloing.
The idea is when you talk to the NPC and get issued your Allied Tags, it starts keeping track of abilities you've used, hp you've cured, damage delt(melee and magic), and a few other things. Every action done will give you credit. And when that battle ends, or when you ask the NPC for a Performance Assesment, they give you Exp/Allied Notes depending on how much you've done. All actions have a max cap per tag. Meaning someone who does 7500 damage to all monsters total, and someone who does 10000 damage to all monsters total will both get the same exp/AN per Performance Assessment.
Additionally you can't just go out there, do a bunch of stuff, come back and get a full reward. Depending on what medal you have you can get more of your reward over a longer period of time. Let's say you do enough to get 1200 exp off of a battle, but you only have the first medal Ribbon of Service I. You'd have to have tags on for 20 min before getting all of that. Return at 15 min, you'll only get 900 exp.
But if you have Steelknight Emblem I, you'll be able to get all 1200 exp after only 15 min. So the driving force behind getting higher awards is getting your exp faster, and being able to buy better gear with allied notes. At the current highest medal, it's possible to get 5400 exp/hour off of Campaign. Further information can be found here. http://wiki.ffxiclopedia.org/wiki/Campaign:_Experience_Point_Calculation_Guide