Elementalists have only 1 weapon slot, but have different skills for each weapon for each of their 4 elemental attunements, effectively giving them 4 weapon slots compared to everyone else's 2. Water attunement tends to have several healing skills for most weapons (staff in particular can put out quite a bit of healing). Air attunement usually has some evasion & control skills, while earth attunement typically has at least 1 defensive skill.
Necromancers have Death Shroud mode, in which damage reduces their Death Shroud life force supply rather than their actual hit points, as well as giving them a set of powerful abilities. They're also the only class with good life steal skills.
Mesmers produce clones and phantasms, effectively spamming the battlefield with decoys.
Each light armor class essentially has some profession mechanics that help them stay alive despite their low armor if they're played in the style appropriate for the class (though this shouldn't be interpreted as the ability to just stand there and take it the way a heavy armor class can).