Some jobs would be grossly overpowered compared to others. For example, Warriors with 11k HP would trounce jobs like Dragoon. Bard would probably be the most overpowered, given they can run while DPSing, BLM would be at a large disadvantage, cant cast while running, low HP.
FFXIV has very limited cross-class abilities, equipment stats are vertical and extremely narrow and generic. PVP in FFXIV would really be extremely boring...DRG vs DRG...they both have the exact same abilities, the guy who activates abilities faster or has better gear wins, not so much even skill really. Again in FFXI, a DRG could sub a mage and have their wyvern use healing breath, or they could sub WAR and provoke to prevent the other player from using abilities, they could third eye to dodge with /SAM,etc.