My main concern is classes not being balance/"OP". The way I am thinking is that you can go fairly high into healing and into tanking then picking up maybe some defensive buffs ten maybe switch to some CC and/or some DPS, tanks wont need a healer and can just outlast/disrupt.
Having played other MMOs with similarly flexible skill systems, equipment usually prevents that. While we'll be able to equip two weapons and switch between them, presumably also switching passives, our armour will stay the same.
All they need to do is something like make spells cost more wearing medium and much more wearing heavy armour compared to light. Or the same might be achieved through enchantments - limited number of enchant slots across all your equipment so you might be able to seriously boost your AC or your healing but not both.