Minor professions slots are gained at levels 20 and 50. You may only join a new profession if all current professions are at least level 35 or you are replacing a profession.
Acrobat[]
Acrobats have a chance to gain a temporary bonus to the combat.proficiency.dodge mskill.
Assassin[]
Assassin's have a chance to gain a temporary bonus to the combat.proficiency.critical mskill.
Fast healer[]
Fast healers have a chance to gain a temporarily increased health regeneration if they are below maximum hit points.
Hoarder[]
Hoarders gain extra carrying capacity.
Lucky caster[]
Lucky casters have a chance to gain a temporary bonus to the combat.proficiency.spellcraft.free cast mskill.
Despite the name, lucky caster may affect guild skills that use SP even if they aren't explicitly spells. Best to inquire before assuming it will work with any specific guild.
Mystic[]
Mystics have a chance to gain a temporarily increased spell point regeneration if they are below maximum spell points.
Tactician[]
Tacticians have a chance to gain a temporary bonus to the combat.armor.penetration mskill.