I'm seeing several posts here that utterly miss the point of this change and it's consequences.So put away your torches and pitchforks and pay attention:
1. Exp will be based on the highest item level equipped in the party/alliance at any time.
2. Mobs that give exp will continue to give exp, which means that a lvl 56 easy prey at 75 will still give exp at 99 and 120. Magian trials that were intended for lvl 75 are still doable at 99, and will still be doable at 99/120.
3. Exp gain in Abyssea will be reduced if you use IL100-120 gear, but the cap will not. So you need to kill a few more critters to cap at 600 xp, but it caps the same regardless.
4. Bonus exp from chests and gov page completion is not affected, as it's BONUS EXP. (Ever wondered why Abyssea NMs do not count towards trial kills of their family/type? Because they only give bonus exp, not regular exp!)
In short, it will not affect you in an appreciable manner. Yes, you get less exp for killing that Fernfelling Chapuli if you use IL120 gear instead of IL99 gear, but you also kill it much faster and you weren't interested in the exp but items it can drop anyway.
And yes, the fault for this lies with the players that complained it was so easy to get exp that they had nothing left to spent their meripos on. SE listened, fixed merit weaponskills in the process, and now exp gain is going under the axe. We did it to ourselves! (Now go kill those opo-opos before they take over the world!)