AoE refers to any party that is set up to damage multiple mobs at once, through either use of polearms (spears) or magic that has an area-effect ('nukes'.) Dwarven warsmiths have a solo AoE attack, the Big Boom (or rather 'had' as folks report this is much too dangerous to employ now.)
Polearm attacks were nerfed in C4 for folks either under lvl 40, or without polearm masteries. However, polearm parties over 40 are still popular, especially in the catacombs/necropolii. My experience with the nerf is that if you have polearm masteries, you still hit as many mobs in C4 as in C3, even if under level 40.
The AoE magical nerf I haven't tested, since I have no nukers yet.
Angro, go to the Screenshots section and download my AOE replay video in the sticky at the top of the section. Its an AOE polearm group in the Necropolis of Sacrifice.
As to the magical side of the aoe nerf I have had a chance to test it. In small groups it's still ok (IE less then 20 mobs total at a time). However, the old massive trains in ABG are now a thing of the past since you will hit no more then 20 mobs (about 4 groups in ABG) with any aoe spell. This is seriously unhealthy for mages since sleep cloud hits only a small ammount of mobs but still puts the same ammount of aggro on the mage. The other part of it being bad is that hate aura also has the same limitation making it hard to keep aggro if you have over 20 mobs. This does not however affect pvp in anyway, the aoe spells still hit all players in the aoe range in pvp.
The only magical skills not affected by the aoe nerf are the summoner skills. These were left out of the nerf on purpose for reasons only ncsoft knows.